logind: handle session leader termination during deserialization more gracefully

logind: handle session leader termination during deserialization more gracefully

We track session leaders by pidfd precisely to make restarts reliable,
as leader exiting before deserialization is somewhat expected.
Such case is already handled gracefully (we'd GC sessions without leader
before kicking off the new cycle), but let's also tweak the log message
a bit to reduce annoyance.

Closes #39556