trip.stoptimesForDate in the GTFS GraphQL API doesn't work without serviceDate for trips that don't happen today

[optionsome]

Expected behavior

Stop times would be always returned.

Observed behavior

Stop times are not returned if the trip doesn't run the date when the information is queried.

This regression seems to originate from #6245.

Version of OTP used (exact commit hash or JAR name)

2.x

Steps to reproduce the problem

1. Run the following query on some trip that isn't on service today:

{
  trip(id: "<some trip>") {
    stoptimesForDate {
      realtimeState
    }
  }
}

[tkalvas]

I made a quick fix in #6480 but I don't really understand what Michael's change means in the larger scale of things, so maybe someone can verify.