Fix display name with empty last name when sorting by last name

[NerLOR]

When sorting by last name, contacts without last name result in , Alice because last and first name are joined with ', ' regardless of their contents. Easy fix: return only first name if last name is empty.

[codecov]

Codecov Report

Merging #2040 (65e4ac9) into master (89adcae) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #2040   +/-   ##
========================================
  Coverage      0.00%   0.00%           
  Complexity      249     249           
========================================
  Files            22      22           
  Lines           748     748           
========================================
  Misses          748     748           

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[skjnldsv]

That makes more sense! :)
Thanks for the feedback!

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/contacts/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22
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