Fix implode error

[akirk]

Without this patch, on the Site Health page you get this error:

 Warning: implode(): Invalid arguments passed in wp-content/plugins/preferred-languages/inc/functions.php on line 999

because preferred_languages_get_user_list() can return an array or false and you cannot implode( ',', false ).

[codecov]

Codecov Report

Merging #590 (9df8ac3) into main (f518e3e) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 9df8ac3 differs from pull request most recent head 66b0b1a. Consider uploading reports for the commit 66b0b1a to get more accurate results

@@           Coverage Diff           @@
##             main     #590   +/-   ##
=======================================
  Coverage   96.94%   96.94%           
=======================================
  Files           5        5           
  Lines         556      556           
  Branches       31       31           
=======================================
  Hits          539      539           
  Misses         17       17           

Flag	Coverage Δ
default	88.38% <100.00%> (ø)
js	100.00% <ø> (ø)
multisite	92.17% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
inc/functions.php	95.70% <100.00%> (ø)

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[swissspidy]

Thanks @akirk!