use string.IsNullOrEmpty(s) to check the input pattern string

`s.IsNullOrEmpty()` throws a NPE without the message.
use `string.IsNullOrEmpty(s)` to check for nullness and emptiness, so the next line of code gets to run.

Author: Palatis

Parakeet/Rule.cs

@@ -214,7 +214,7 @@ public class CaseInvariantStringRule : Rule
 
         public CaseInvariantStringRule(string s)
         {
-            if (s.IsNullOrEmpty())
+            if (string.IsNullOrEmpty(s))
                 throw new ArgumentException("Pattern must be non-empty", nameof(s));
             Pattern = s;
         }