LutFinder: do not expect a valid app directory to be present

If there is one, we should look into its luts/ directory. If there
is none, however, we should still look into the .jar files' luts/
directory without complaining too much.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
We must not ask a String to do a File's job

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
wip