Running lfric_atm in coupled mode with only rivers tries to read ocean and seaice fields

### Version

main

### Are there any linked Issues or Pull Requests?

_No response_

### What happened?

I am trying to run lfric_atm coupled to the river routing model, without oceans or sea ice at vn3.2. The model fell over trying to access the field `lf_ocn_sst` in what I believe is routine `interfaces/coupled_interface/source/algorithm/process_recv_fields_2d_mod.x90`. There are some lines in this routine that should be protected by `l_couple_ocean` or `l_couple_seaice` in case the model is not coupled to the ocean or seaice models.

### Relevant log output

```shell
20260817143636.586+0000:P000:INFO : lfric_atm: Couple flag l_esm_couple_test: F
20260817143636.586+0000:P000:S1:INFO : Coupling timestep: 0
20260817143636.592+0000:P000:S1:ERROR: get_field: No 32-bit field [lf_ocn_sst] in field collection: cpl_rcv_2d
```