Don't allow init_interp_fill_missing_with_natveg to work on urban landunits

With #3110 we figured out that init_interp_fill_missing_with_natveg can NOT be used with urban landunits. So we should change the code so that this namelist item doesn't operate for urban land units.

This came with this observation:
> One thing I glean from this is that init_interp_fill_missing_with_natveg -- shouldn't be allowed for urban landunits. It obviously works for all veg landunits, and might work for wetland, glacier or lake. But, we should disallow it for urban -- they are just too fundamentally different.
>

_Originally posted by @ekluzek in [#3107](https://github.com/ESCOMP/CTSM/issues/3107#issuecomment-2855510766)_

Definition of done:
- [x] Have the Fortran code abort in this case
- [ ] Have build-namelist abort for this case