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.

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