dbus: permit seeing process list of units whose unit files are missing

dbus: permit seeing process list of units whose unit files are missing

Previously, we'd refuse the GetUnitProcesses() bus call if the unit file
couldn't be loaded. Which is wrong, as admins should be able to inspect
services whose unit files was deleted. Change this logic, so that we
permit introspecting the processes of any unit that is loaded,
regardless if it has a unit file or not.

(Note that we won't load unit files in GetUnitProcess(), but only
operate on already loaded ones. That's because only loaded units can
have processes — as that's how our GC logic works — and hence loading
the unit just for the process tree is pointless, as it would be empty).

See: #4995