Have a way to explicitly communicate that a trip does not have any headsigns

### Describe the problem

In some small and rural agencies, the vehicles they operate don't have any headsigns and may merely have a vehicle with the overall transit agency name present. As promoted in #485, the headsigns field is being said the be "recommended". Therefore this creates a tension in assuming that data is missing when the headsigns data is not included.

### Use cases

Two examples from California are shown below:

Morro Bay Transit:


Dana Point Trolley:


I would assume there are other places throughout the world where headsigns aren't used or don't exist.

### Proposed solution

There should be a some language within the specification that specifically notes how to note that headsign information is excluded. The spec could be rewritten so that when headsigns are not included, this constitutes an acknowledgment that headsigns aren't used for the trip in question. Otherwise, a flag could be added to note that headsings aren't used for a particular trip.

### Additional information

_No response_