core/unit: ignore dropins for masked units completely when checking need_reload

core/unit: ignore dropins for masked units completely when checking need_reload

Follow-up for 19a44dfe4525ab01caf593a9c2beada4b412910d

If a drop-in is set from upper level, e.g. global unit_type.d/,
even if a unit is masked, its dropin_paths would still be partially
populated. However, unit_need_daemon_reload() would always
compare u->dropin_paths with empty strv in case of masked units,
resulting in it always returning true. Instead, let's ignore
dropins entirely here.

Fixes #33672