Don't check whether an empty filename exists

Summary:
Since 5.10 (commit 0dee566e98 in qtbase) Qt complains if we make a
system call with an empty filename (with a "Empty filename passed to
function" runtime warning).

There is no reason to call exists() if we already know that a filename
is empty. While at it, port to QFileInfo::exists() which is faster.

Test Plan: Together with D8696, fixes all the runtime warnings when running tests/kfilewidgettest_gui.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: apol, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8697