It was done in one case (CommandLine) and not the other (Activate), making
things very inconsistent. An empty list of arguments makes QCommandLineParser
fail with a warning (or even exit, when using process() instead of parse()),
so apps would warn or exit unless they had an explicit check for empty list of args.

Since this is a (necessary) behaviour change, I had a look at all the users of KDBusService::Unique I could find.

• khelpcenter: was exiting due to QCLP::process when started twice, fixed by calling parse() instead, and fixed by this commit.
• konsole: was unconditionally prepending the exe name, but fortunately ignoring positional args, so no behaviour change. Debug output (independent from this commit) : Konsole::Application::slotActivateRequested: ("/usr/bin/konsole", "konsole", "--new-tab")
• plasmawindowed: early exit when called without arguments, so not affected.
• plasma-discover: got a bug report for the QCLP assert (367944), fixed by early return, now unnecessary.
• kfontinst, pim-data-exporter: early return if args list empty, will be ok with going in the if now.
• kleopatra, kalarm: had a check (workaround) for the case of empty argument list, now unnecessary.
• kwalletmanager, bluedevil, systemsettings, krunner, plasma-nm-editor: no parsing of arguments in the slot connected to ActivateRequested, OK
• about 15-20 others: no handling of activateRequested, OK

• kde-print-queue: BUGGY. On second run, it tries to print the queue "kde-print-queue" as well. Can only be fixed by skipping first arg, after relying on this fix.