Chapter13.15 bug fix (#794)

## Summary
This pull request addresses an issue in
`Chapter13.15.ParameterlessStatementLambdas.cs` where `getUserInput` was
only taking white space as input. The method has been updated to ensure
that only valid, non-whitespace strings are accepted.

## Changes Made
- Removed the Not (`!`) on the while loop condition in `getUserInput` to
properly handle whitespace inputs.
- Updated 'Main' to call `Console.WriteLine(input);` for testing.
- Added unit test to verify correct behavior for valid inputs.

---------

Co-authored-by: Benjamin Michaelis <[email protected]>

Errata.md

@@ -40,4 +40,5 @@ Zhou Jing | 23 | 1080 | Change "DWORD flProtect); // The type of memory allocati
 Zhou Jing | 22 | 1065 - 1066 | Changed 'Thread' to 'Task' and "Application exiting" to "Application shutting down"
 Zhou Jing | 4 | 161 | Fix `input < 9` to `input < 0` in Listing 4.24
 Zhou Jing    | 4 | 119 | Show inconsistent size multi-dimensional array in listing 3.16
-Zhou Jing | 3  | 114 | Replace `second` with `third` in "// Retrieve third item from the end (Python)"
+Zhou Jing | 3  | 114 | Replace `second` with `third` in "// Retrieve third item from the end (Python)"
+Tyler Woody | 13 | 702 | Remove the `!` negation in `string.IsNullOrWhiteSpace(input)` in the while loop to properly allow looping

src/Chapter13.Tests/Listing13.15.Tests.cs

@@ -0,0 +1,15 @@
+namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter13.Listing13_15.Tests;
+
+[TestClass]
+public class ProgramTests
+{
+    [TestMethod]
+    public void MainTest()
+    {
+        const string expected = "<<ValidInput>>ValidInput";
+
+        IntelliTect.TestTools.Console.ConsoleAssert.Expect(
+            expected, Program.Main
+        );
+    }
+}

src/Chapter13/Listing13.15.ParameterlessStatementLambdas.cs

@@ -17,11 +17,12 @@ public static void Main()
                 {
                     input = Console.ReadLine();
                 }
-                while(!string.IsNullOrWhiteSpace(input));
+                while(string.IsNullOrWhiteSpace(input));
                 return input!;
             };
         //...
         #endregion INCLUDE
-        getUserInput();
+        string input = getUserInput();
+        Console.WriteLine(input);
     }
 }