(#555) Clarify ITypeResolver returns null

and does not throw on unresolvable types.
Also changed the TypeResolverAdapter to adhere
to those expectations and removed the now no longer
needed try-catch from CommandPropertyBinder.

examples/Cli/Injection/Infrastructure/TypeResolver.cs

@@ -15,7 +15,12 @@ namespace Spectre.Console.Examples
 
         public object Resolve(Type type)
         {
-            return _provider.GetRequiredService(type);
+            if (type == null)
+            {
+                return null;
+            }
+
+            return _provider.GetService(type);
         }
 
         public void Dispose()