Fix #16187: error caused by array_combine() with PHP<5.4

array_combine() function behavior was changed in PHP 5.4 [1]; in earlier
versions it would issue a warning and return false when working with
empty arrays, which then causes Mantis to trigger an application error.

This is a regression introduced by cc7703acc8d05f253d52a152a7d8cd0c1c43815d

[1] http://php.net/function.array-combine.php