flatpak-coredumpctl: Remove nargs="?" from -m option

-m with no argument currently results in coredumpctl_matches having a
value of "", which is exactly the same as if it weren't used at all. As
far as I can tell, allowing this does nothing useful, only making the
usage slightly less clear.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f