Fix nullable warnings in tests

Author: rameel

tests/Rmastack.ExpressionParser.Tests/ExpressionParserTest.cs

@@ -114,7 +114,7 @@ private static List<object[]> GetResource(string resourceName)
 
     public class UserInfo
     {
-        public string Name { get; set; }
-        public string LastName { get; set; }
+        public string? Name { get; set; }
+        public string? LastName { get; set; }
     }
 }