Make sure that output produce is never null

Author: DigitalFlow

src/lib/Xrm.Oss.XTL.Interpreter/XTLInterpreter.cs

@@ -437,7 +437,7 @@ public string Produce()
 
             var output = Formula(new Dictionary<string, ValueExpression> { });
 
-            return output?.Text;
+            return output?.Text ?? string.Empty;
         }
     }
 }