(version 5.13/5.14) Search Builder syntax error when "Groups" "is empty"
This is a recent bug introduced by the fix to a different search issue, issue #726.

A permanent fix will need to be found that doesn't break either scenario.  However, to resolve your immediate issue, you can open file CRM/Contact/BAO/Query.php and delete line 3037.

I've reported this issue as #1048.