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.

(cherry picked from commit e1e74614)
(cherry picked from commit 252eee75)