[TASK] RTE: Avoid use of Ext.isEmpty

Streamline code.

Releases: master
Resolves: #62619
Change-Id: I4218c0ae03289d71cba2c40547d0bc4f7a71868b
Reviewed-on: http://review.typo3.org/33709
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>