Investigate differing cell measures for ond/grid-T variables

While conducting tests using suite u-dw498 under experiment esm-piControl and model UKESM1-3-LL, the following error arose for multiple variables under the stream ond/grid-T.

```
Unable to produce MIP requested variable "zostoga_tavg-u-hm-sea" for "CMIP7_ocean": failed to concatenate into a single cube. Cell measures differ: cell_area != < None >
```

Example variable: `ocean/zostoga_tavg-u-hm-sea@day:ond/grid-T`

It is possible that this may be due to missing/out of date ancillary files. This should be re tested once the provision of UKESM1-3 ancillaries has been completed #698. If the issue persists, this should be investigated.

First check whether UKESM1-3 ancillaries from #698 are complete and rerun suite u-dw498 under esm-piControl with UKESM1-3-LL. Re-test the example variable ocean/zostoga_tavg-u-hm-sea@day:ond/grid-T; done means determining whether the cell_area versus None mismatch persists and, if so, identifying its cause.

Written by the indexing model from the issue text.