[BUGFIX] Empty text with non-localizable label key

When method $GLOBALS['TSFE']->sL() is called with a non-localizable
label key (not starting with "LLL:"), an empty string is returned
instead of the label key itself.

This is a breaking change with previous versions of TYPO3.

Change-Id: I2847e51fd479d9636ac815c820b2e63cf449d6a7
Fixes: #31347
Releases: 4.6, 4.7
Reviewed-on: http://review.typo3.org/6364
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf