[BUGFIX] Remove empty values in pageOverlayFields

$GLOBALS['TYPO3_CONF_VARS']['FE']['pageOverlayFields'] might contain
empty values. Remove those.

Resolves: #62068
Releases: master, 6.2
Change-Id: I79337d9d8dca17fc840a331c8839147054d1b3f0
Reviewed-on: http://review.typo3.org/33192
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>