core: don't create Requires for workdir if "missing ok"

core: don't create Requires for workdir if "missing ok"

Don't add an implicit RequiresMountsFor depenency for the
WorkingDirectory of a unit if the "-" character was used to
indicate that "a missing working directory is not considered fatal"
(see systemd.exec(5)). Otherwise systemd might fail the unit
because of missing dependencies.