Added nullable helper for IfNullOrWhiteSpace (#12979)

* Added nullable helper for IfNullOrWhiteSpace

* Remove for str as that one could be an empty string

src/Umbraco.Core/Extensions/StringExtensions.cs

@@ -419,7 +419,8 @@ public static class StringExtensions
     ///     returns <see langword="false" />.
     /// </returns>
     public static bool IsNullOrWhiteSpace(this string? value) => string.IsNullOrWhiteSpace(value);
-
+    
+    [return: NotNullIfNotNull("defaultValue")]
     public static string? IfNullOrWhiteSpace(this string? str, string? defaultValue) =>
         str.IsNullOrWhiteSpace() ? defaultValue : str;