Don't cache the gadget-defintion in case no gadgets were found

After some investigation I could only find a few unlike
possibilities which eventually could lead to the issues
described in the bug. In the end I decided that it would
be the easiest and probably most robust solution to simply
don't cache in case no gadgets were found, even if the
MediaWiki page exists.
This shouldn't cause any performance issues as we already
don't cache in case no gadgets could be found in all paths
of this function except after trying to parse the gadget
definition.

Bug: 37228
Change-Id: I3092bcb162d032282fbe263c7f14f4d1ce9163ab