Fix DefaultValue for FileInfo and DirectoryInfo (#1238)

Commit d3f4f5f208 introduced automatic conversion to FileInfo and DirectoryInfo but failed to properly handle the conversion if the value comes from the [DefaultValue] attribute.

Using both `var (converter, stringConstructor) = GetConverter(...)` and `var (converter, _) = GetConverter(...)` should have been a red flag!

test/Spectre.Console.Cli.Tests/Data/Settings/HorseSettings.cs

@@ -8,6 +8,7 @@ public class HorseSettings : MammalSettings
     public DayOfWeek Day { get; set; }
 
     [CommandOption("--file")]
+    [DefaultValue("food.txt")]
     public FileInfo File { get; set; }
 
     [CommandOption("--directory")]

test/Spectre.Console.Cli.Tests/Unit/CommandAppTests.cs

@@ -989,6 +989,7 @@ public sealed partial class CommandAppTests
         {
             horse.Legs.ShouldBe(4);
             horse.Name.ShouldBe("Arkle");
+            horse.File.Name.ShouldBe("food.txt");
         });
     }