[BUGFIX] Don't scan for FCE registrations when FluidContent is installed
[TASK] Throw Exception if attempting to getFiles empty path

Before: PHP fatal error, no stack dump.
After: Exception with stack dump to help devs locate source of problem.
[BUGFIX] Avoid 6.0 crash on saving elements by avoiding cache clear

Internally this has the result that the TS for FCEs is not rebuilt at a time that is bad for the TYPO3 internals which are being queried to detect FCEs. The FCE detection is currently part of both EXT:fed and EXT:fluidcontent but will be rebuilt in EXT:fluidcontent, from the ground up. At that point the official recommendation is to not use FED unless combined with EXT:fluidcontent (and EXT:fluidpages if desired).

Fixes: #10