Lessen log spam by not checking for existence of file with empty name

Summary:
Calls to `QFile::exists()` while starting a Plasma session
resulted in `Empty filename passed to function` being logged
since Qt 5.10 (0dee566e98f5 in qtbase).

This can be avoided by skipping the check for empty filenames, which
would have returned `false` in any case. Similarly to what was done in
R241:549fc81e8e8a, also port to the faster `QFileInfo::exists()`.

Note this does not simply hide a deeper issue, i.e. the actual cause
resulting in the empty variable, because `libpath.isEmpty()` will be
handled explicitly later on. In the case under investigation
`/usr/lib64/libexec/org_kde_powerdevil` did not match
`/usr/lib64/libexec/kf5`, but even if it was in that subfolder there
would have been no corresponding `libkdeinit5_` anyway.

CCBUG: 388611

Test Plan:
Grep session log for "Empty filename passed to function" after
restarting the Plasma session and compare before/after.

Reviewers: #frameworks

Subscribers: dfaure, kde-frameworks-devel, dhaumann

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D9987