Fix issue where null would be passed to explode

Marijn van Wezelrequested to merge
null-passed-to-explode-deprecation into master

This fixes the following deprecation warning:

Deprecated
: explode(): Passing null to parameter #2 ($string) of type string is deprecated in
/var/www/html/extensions/WSArrays/src/GlobalFunctions.class.php
on line 434

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