Allow passing a nullable style in DefaultValueStyle() and ChoicesStyle()

This will allow to slightly simplify the implementation of #1210

See also related discussion on #1349 (comment)

src/Spectre.Console/Prompts/TextPrompt.cs

@@ -75,12 +75,12 @@ public sealed class TextPrompt<T> : IPrompt<T>, IHasCulture
     public Func<T, ValidationResult>? Validator { get; set; }
 
     /// <summary>
-    /// Gets or sets the style in which the default value is displayed.
+    /// Gets or sets the style in which the default value is displayed. Defaults to green when <see langword="null"/>.
     /// </summary>
     public Style? DefaultValueStyle { get; set; }
 
     /// <summary>
-    /// Gets or sets the style in which the list of choices is displayed.
+    /// Gets or sets the style in which the list of choices is displayed. Defaults to blue when <see langword="null"/>.
     /// </summary>
     public Style? ChoicesStyle { get; set; }
 

src/Spectre.Console/Prompts/TextPromptExtensions.cs

@@ -330,20 +330,15 @@ public static TextPrompt<T> WithConverter<T>(this TextPrompt<T> obj, Func<T, str
     /// </summary>
     /// <typeparam name="T">The prompt result type.</typeparam>
     /// <param name="obj">The prompt.</param>
-    /// <param name="style">The default value style.</param>
+    /// <param name="style">The default value style or <see langword="null"/> to use the default style (green).</param>
     /// <returns>The same instance so that multiple calls can be chained.</returns>
-    public static TextPrompt<T> DefaultValueStyle<T>(this TextPrompt<T> obj, Style style)
+    public static TextPrompt<T> DefaultValueStyle<T>(this TextPrompt<T> obj, Style? style)
     {
         if (obj is null)
         {
             throw new ArgumentNullException(nameof(obj));
         }
 
-        if (style is null)
-        {
-            throw new ArgumentNullException(nameof(style));
-        }
-
         obj.DefaultValueStyle = style;
         return obj;
     }
@@ -353,20 +348,15 @@ public static TextPrompt<T> DefaultValueStyle<T>(this TextPrompt<T> obj, Style s
     /// </summary>
     /// <typeparam name="T">The prompt result type.</typeparam>
     /// <param name="obj">The prompt.</param>
-    /// <param name="style">The style to use for displaying the choices.</param>
+    /// <param name="style">The style to use for displaying the choices or <see langword="null"/> to use the default style (blue).</param>
     /// <returns>The same instance so that multiple calls can be chained.</returns>
-    public static TextPrompt<T> ChoicesStyle<T>(this TextPrompt<T> obj, Style style)
+    public static TextPrompt<T> ChoicesStyle<T>(this TextPrompt<T> obj, Style? style)
     {
         if (obj is null)
         {
             throw new ArgumentNullException(nameof(obj));
         }
 
-        if (style is null)
-        {
-            throw new ArgumentNullException(nameof(style));
-        }
-
         obj.ChoicesStyle = style;
         return obj;
     }