Fix warning: mimeType "x-scheme-handler/file" not found

commit b5af9b4eb021f772ce27b466409f6bd1efd72969 ported KIO::DesktopExecParser::hasSchemeHandler
from KMimeTypeTrader to KApplicationTrader, but it turns out there was a slight difference
between the two: KMimeTypeTrader had two if()s for silencing the x-scheme-handler/* warnings,
while KApplicationTrader had only one. Maybe I misread the code back then and failed to see
the "mime = mimeType" line (rather than return) which makes us proceed further down after
QMimeDatabase says the mimetype doesn't exist.

BUG: 442721
FIXED-IN: 5.116
(cherry picked from commit d0f416ea)