CMIP7: UKCM2: Investigate maximum category thickness' with 'None' values

[mo-laurenboon]

During testing for UKCM2, a set of variables triggered the error: "('category maximum thickness', None)". This should be investigated further and resolved where possible.

The affected variables are listed below:

Grid	Variable	Mapping Information
mip_convert_ind_ocean-native-icemod	siitdconc_tavg-u-hxy-si	1672
mip_convert_ind_ocean-native-icemod	siitdsnconc_tavg-u-hxy-si	1673
mip_convert_ind_ocean-native-icemod	siitdsnthick_tavg-u-hxy-si	1674
mip_convert_ind_ocean-native-icemod	siitdthick_tavg-u-hxy-si	1675
mip_convert_inm_ocean-native-icemod	siitdconc_tavg-u-hxy-si	1747
mip_convert_inm_ocean-native-icemod	siitdsnconc_tavg-u-hxy-si	1748
mip_convert_inm_ocean-native-icemod	siitdthick_tavg-u-hxy-si	1750