[mod] results: don't crash when an engine don't have a category

According to
https://github.com/searx/searx/blob/820b468bfe96f693d60ce06f1e78af51f00deefc/searx/engines/__init__.py#L87-L88

an engine can have no category at all.

Without this commit, searx raise an exception in searx/results.py

Note: in this case, the engine is not shown in the preferences.