oomd: avoid unnecessary wake-ups for ManagedOOMSwap

If there are no "ManagedOOMSwap" entries to monitor, then the event source
can be completely disabled. This scenario appears to be common and avoiding
the wake-ups can save idle energy consumption.

This was discovered while using Sysprof for various GNOME 45 performance
tuning. systemd-oomd goes from waking up a few times a second to no
wake-ups helping keep a laptop in deep(er) sleep.

Signed-off-by: Christian Hergert <chergert@redhat.com>
(cherry picked from commit b63beb4d283438be2b0f5c62757ab0db7dee8d65)