Don't erase the prompt text when backspacing on a secret prompt with a null mask.

src/Spectre.Console/Extensions/AnsiConsoleExtensions.Input.cs

@@ -53,7 +53,11 @@ public static partial class AnsiConsoleExtensions
                 if (text.Length > 0)
                 {
                     text = text.Substring(0, text.Length - 1);
-                    console.Write("\b \b");
+
+                    if (mask != null)
+                    {
+                        console.Write("\b \b");
+                    }
                 }
 
                 continue;

test/Spectre.Console.Tests/Expectations/Prompts/Text/SecretValueBackspaceNullMask.Output.verified.txt

@@ -0,0 +1 @@
+Favorite fruit? 

test/Spectre.Console.Tests/Unit/Prompts/TextPromptTests.cs

@@ -248,6 +248,25 @@ public sealed class TextPromptTests
         return Verifier.Verify(console.Output);
     }
 
+    [Fact]
+    [Expectation("SecretValueBackspaceNullMask")]
+    public Task Should_Not_Erase_Prompt_Text_On_Backspace_If_Prompt_Is_Secret_And_Mask_Is_Null()
+    {
+        // Given
+        var console = new TestConsole();
+        console.Input.PushText("Bananas");
+        console.Input.PushKey(ConsoleKey.Backspace);
+        console.Input.PushKey(ConsoleKey.Enter);
+
+        // When
+        console.Prompt(
+            new TextPrompt<string>("Favorite fruit?")
+                .Secret(null));
+
+        // Then
+        return Verifier.Verify(console.Output);
+    }
+
     [Fact]
     [Expectation("SecretDefaultValueCustomMask")]
     public Task Should_Choose_Custom_Masked_Default_Value_If_Nothing_Is_Entered_And_Prompt_Is_Secret_And_Mask_Is_Custom()