If a sample result is null, then currently the Response Assertion fails the sample even if the "Not" checkbox is used. A null response could be treated the same as an empty response in this case.
Fixed in SVN trunk: URL: http://svn.apache.org/viewvc?rev=701680&view=rev Log: Bug 45904 - Allow 'Not' Response Assertion to succeed with null sample
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/2157