Fix assert when deleting a folder in kmail

MessageList::Pane::Private::setCurrentFolder ends up calling
StorageModel::savePreSelectedMessage for the old folder, usually in
order to save the current message in that folder. But if we just deleted
that folder (and that's the reason we're "moving away" from it), then
it's not selected anymore and StorageModel::id() returns empty.

Just return early from savePreSelectedMessage in that case.
1 job for v20.11.90 in 28 minutes and 35 seconds (queued for 440 minutes and 38 seconds)
latest
Status Job ID Name Coverage
  Build
passed #80257
build_clazy_clang_tidy

00:28:35