ReplaceWithStringIsNullOrEmptyIssue now detects `x == string.Empty`

Previously, the issue would detect `x == null || x == ""` but not `x == null || x == string.Empty`.
This commit changes that.

This is a very simple change. For instance, it does not detect `x == System.String.Empty` and `x == SomeConstThatHappensToBeAnEmptyString`.