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.

src/core/unit.c

@@ -964,7 +964,7 @@ int unit_add_exec_dependencies(Unit *u, ExecContext *c) {
         assert(u);
         assert(c);
 
-        if (c->working_directory) {
+        if (c->working_directory && !c->working_directory_missing_ok) {
                 r = unit_require_mounts_for(u, c->working_directory, UNIT_DEPENDENCY_FILE);
                 if (r < 0)
                         return r;