[CLEANUP] Replace count with empty in EXT:extensionmanager

Replace occurences of count() with matching empty() pendants.

Resolves: #67781
Releases: master
Change-Id: I4279ab12911ff7aa82c42e5a8bc2875c6e41f299
Reviewed-on: http://review.typo3.org/40689


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>