Bug 35570: (QA follow-up): Update UNIMARC author to 200$f

Bug 35570: (QA follow-up): Update UNIMARC author to 200$f

Reworked the logic here a bit to ensure the following scenario for UNIMARC:
If both title and author exist, the result is:
200$aTitle$fAuthor

In the previous logic, it would become:
200$aTitle
200$fAuthor

I'm assuming the desired result is the former and not the latter.
Test following the same test plan, but try having only author, or only title, and verify that the resulted MARC data is as expected.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>