Filter out undefined return values from SetMap
Filter out undefined return values from SetMap

This is a workaround for a content bug. We shouldnt
really have articles tagged with tags that dont
appear in any set, but in the case where we do,
SetMap.get_set_for_tag will return undefined.

This filters out those undefined values.

https://phabricator.endlessm.com/T10717

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib