do not fail if fedcats is empty in config

[pvgenuchten]

Overview

in python, if foo['bar'] is None:

foo.get('bar',[]) == None

therefore add a None check, before calling it, probable there is a more efficient way to do it...

Related Issue / Discussion

#947

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• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
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[pvgenuchten]

updated as suggested @tomkralidis