[CLEANUP] Replace strlen() with === for zero length check

It is faster to compare a string with === '' to find out if it's empty
than to run strlen() on it.

The replacement rules are applied as follows:
 *    if (strlen($str))
   => if ((string)$str !== '')
 *    if (!is_string($str) || strlen($str) === 0)
   => if (!is_string($str) || $str === '')
 * If it can be seen easily that $str is a string,
   the typecast is omitted.

Change-Id: I888d70e42f925bd57ad23b873b72d6a38acb39ef
Resolves: #54091
Releases: master
Reviewed-on: http://review.typo3.org/27794


Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>