Update MacroContent.cs to use is operator for comparison to null

Co-Authored-By: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Core/Macros/MacroContent.cs

@@ -12,7 +12,7 @@ namespace Umbraco.Web.Macros
         public DateTime Date { get; set; } = DateTime.Now;
 
         // a value indicating whether the content is empty
-        public bool IsEmpty => Text == null;
+        public bool IsEmpty => Text is null;
 
         // gets an empty macro content
         public static MacroContent Empty { get; } = new MacroContent();