Snapshot runs (null dycore) should always pass is_first_timestep = .false. to physics

### What happened?

CAM does not write out snapshots until the second timestep.
This means that for snapshots driving CAM-SIMA, it is never truly the first timestep for these runs as far as physics is concerned.
This is a corner case found during CCPPization of the UW PBL scheme, which has special handling for the first model timestep.

The null dycore should be modified to always override `is_first_timestep = .false.` when it timesteps

### What are the steps to reproduce the bug?

Output `is_first_timestep` in any physics scheme when running a snapshot run of CAM-SIMA

### What CAM-SIMA hash were you using?

04a0b5aa68fdfac9152fcd785ccba9894c0bf2e1

### What machine were you running CAM-SIMA on?

CISL machine (e.g. cheyenne)

### What compiler were you using?

GNU

### Path to a case directory, if applicable

_No response_

### Will you be addressing this bug yourself?

Yes

### Extra info

_No response_