Make mapcat be a transducer if no collections are provided

[jonasseglare]

This resolves issue #607, so that mapcat will return a transducer in case no collections are provided:

(def predicate-keyword-id-map (into {}
                                    (mapcat (fn [[keyword ids]]
                                              (map (fn [id] [id keyword]) ids)))
                                    predicate-id-keyword-map))

Please answer the following questions and leave the below in as part of your PR.

• This PR corresponds to an issue with a clear problem statement.

• This PR contains a test to prevent against future regressions

• I have updated the CHANGELOG.md file with a description of the addressed issue.

[borkdude]

Awesome! Can you also update the changelog?

[jonasseglare]

Thanks, changelog has been updated.