develwarn: do not emit warning if "config" is unspecified

Previously, if the develwarn call site did not specify the category of warning,
and devel.all-warnings was False, it would emit the warning.  If it was
intended that this emit a warning if config is unspecified, I would have
expected a comment, so I assumed this was unintentional and am changing the
behavior.

Differential Revision: https://phab.mercurial-scm.org/D1494

mercurial/ui.py

@@ -1603,7 +1603,7 b' class ui(object):'
         stack.
         """
         if not self.configbool('devel', 'all-warnings'):
-            if config is not None and not self.configbool('devel', config):
+            if config is None or not self.configbool('devel', config):
                 return
         msg = 'devel-warn: ' + msg
         stacklevel += 1 # get in develwarn