Merge branch...

Merge branch '6452-uncaught-typeerror-array_merge-argument-1-must-be-of-type-array-bool-given-in' into 'develop'

Resolve "Uncaught TypeError: array_merge(): Argument 1 must be of type array, bool given in shelluser_base_plugin.inc.php:480"

Closes #6452

See merge request ispconfig/ispconfig3!1689