Make internal_heat optional so it works when geothermal heating is disable in MOM6

[yichengt900]

As @andrew-c-ross originally reported here, COBALT uses the internal (geothermal) heat argument to calculate an iron flux:

ocean_BGC/generic_tracers/generic_COBALT.F90

Line 7850 in 011b724

cobalt%ffe_geotherm(i,j) = cobalt%ffe_geotherm_ratio*internal_heat(i,j)*4184.0/dt

However, the internal_heat is a null pointer by default in MOM6 and is only allocated when DO_GEOTHERMAL = True is set in MOM_input/MOM_override:
https://github.com/NOAA-GFDL/MOM6/blob/2c1a9d32fce72828b7091e6f623c0ec20069e637/src/core/MOM.F90#L2812

We will encounter a segmentation fault in COBALT if the user accidentally sets DO_GEOTHERMAL = false. This PR, proposed by @andrew-c-ross, addresses this issue by checking if internal_heat is a null pointer (by making it an optional argument in generic_COBALT_update_from_source subroutine and checking if it is present) and assuming zero internal heating/iron flux if it is null.

[andrew-c-ross]

Thanks Yi-Cheng!

[charliestock]

Looks good Yi-Cheng. Thank you!