Enable nullable reference types

Closes #36

src/Spectre.Console/Internal/Colors/ColorTable.cs

@@ -45,7 +45,7 @@ namespace Spectre.Console.Internal
             return ColorPalette.EightBit[number];
         }
 
-        public static string GetName(int number)
+        public static string? GetName(int number)
         {
             _nameLookup.TryGetValue(number, out var name);
             return name;

src/Spectre.Console/Internal/Extensions/StringExtensions.cs

@@ -17,14 +17,9 @@ namespace Spectre.Console.Internal
 
         public static string NormalizeLineEndings(this string text, bool native = false)
         {
-            if (text == null)
-            {
-                return null;
-            }
-
-            var normalized = text?.Replace("\r\n", "\n")
-                ?.Replace("\r", string.Empty);
+            text ??= string.Empty;
 
+            var normalized = text?.Replace("\r\n", "\n")?.Replace("\r", string.Empty) ?? string.Empty;
             if (native && !_alreadyNormalized)
             {
                 normalized = normalized.Replace("\n", Environment.NewLine);
@@ -35,7 +30,8 @@ namespace Spectre.Console.Internal
 
         public static string[] SplitLines(this string text)
         {
-            return text.NormalizeLineEndings().Split(new[] { '\n' }, StringSplitOptions.None);
+            var result = text?.NormalizeLineEndings()?.Split(new[] { '\n' }, StringSplitOptions.None);
+            return result ?? Array.Empty<string>();
         }
     }
 }

src/Spectre.Console/Internal/Text/Markup/MarkupParser.cs

@@ -6,7 +6,7 @@ namespace Spectre.Console.Internal
 {
     internal static class MarkupParser
     {
-        public static Text Parse(string text, Style style = null)
+        public static Text Parse(string text, Style? style = null)
         {
             style ??= Style.Plain;
 
@@ -18,6 +18,10 @@ namespace Spectre.Console.Internal
             while (tokenizer.MoveNext())
             {
                 var token = tokenizer.Current;
+                if (token == null)
+                {
+                    break;
+                }
 
                 if (token.Kind == MarkupTokenKind.Open)
                 {

src/Spectre.Console/Internal/Text/Markup/MarkupTokenizer.cs

@@ -7,7 +7,7 @@ namespace Spectre.Console.Internal
     {
         private readonly StringBuffer _reader;
 
-        public MarkupToken Current { get; private set; }
+        public MarkupToken? Current { get; private set; }
 
         public MarkupTokenizer(string text)
         {

src/Spectre.Console/Internal/Text/StringBuffer.cs

@@ -1,10 +1,12 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 
 namespace Spectre.Console.Internal
 {
     internal sealed class StringBuffer : IDisposable
     {
+        [SuppressMessage("Usage", "CA2213:Disposable fields should be disposed", Justification = "False positive")]
         private readonly StringReader _reader;
         private readonly int _length;
 

src/Spectre.Console/Internal/Text/StyleParser.cs

@@ -14,16 +14,23 @@ namespace Spectre.Console.Internal
                 throw new InvalidOperationException(error);
             }
 
+            if (style == null)
+            {
+                // This should not happen, but we need to please the compiler
+                // which cannot know that style isn't null here.
+                throw new InvalidOperationException("Could not parse style.");
+            }
+
             return style;
         }
 
-        public static bool TryParse(string text, out Style style)
+        public static bool TryParse(string text, out Style? style)
         {
             style = Parse(text, out var error);
             return error == null;
         }
 
-        private static Style Parse(string text, out string error)
+        private static Style? Parse(string text, out string? error)
         {
             var effectiveDecoration = (Decoration?)null;
             var effectiveForeground = (Color?)null;
@@ -113,11 +120,11 @@ namespace Spectre.Console.Internal
         }
 
         [SuppressMessage("Design", "CA1031:Do not catch general exception types")]
-        private static Color? ParseHexColor(string hex, out string error)
+        private static Color? ParseHexColor(string hex, out string? error)
         {
             error = null;
 
-            hex = hex ?? string.Empty;
+            hex ??= string.Empty;
             hex = hex.Replace("#", string.Empty).Trim();
 
             try
@@ -151,11 +158,12 @@ namespace Spectre.Console.Internal
         }
 
         [SuppressMessage("Design", "CA1031:Do not catch general exception types")]
-        private static Color? ParseRgbColor(string rgb, out string error)
+        private static Color? ParseRgbColor(string rgb, out string? error)
         {
             try
             {
                 error = null;
+
                 var normalized = rgb ?? string.Empty;
                 if (normalized.Length >= 3)
                 {

src/Spectre.Console/Internal/Utilities/Ratio.cs

@@ -40,7 +40,7 @@ namespace Spectre.Console.Internal
             return result;
         }
 
-        public static List<int> Distribute(int total, List<int> ratios, List<int> minimums = null)
+        public static List<int> Distribute(int total, List<int> ratios, List<int>? minimums = null)
         {
             if (minimums != null)
             {