In the situation you present, $RequestorEmail contains an empty string.
In your 1st test, since the variable $RequestorEmail is NOT $null (because it holds an empty string), and the string DOES contain an empty string, both conditions fail so the result is FALSE.
In your 2nd test, the variable $RequestorEmail IS empty and the IsNullOrEmpty is true, but you're negating that with the -not operator, so the result is FALSE.
What results where you expecting? And why?