XIOS3 missing value

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

_No response_

### Brief description

A specific feature was put into XIOS2 to adapt how `_FillValue` is written out:
https://gitlab.in2p3.fr/ipsl/projets/xios-projects/xios/-/commit/613dc4912761a20ae0d131c3717e6a98b90af2d7

but this is not part of the XIOS3 feature set.
https://gitlab.in2p3.fr/ipsl/projets/xios-projects/xios/-/blob/main/src/io/nc4_data_output.cpp?ref_type=heads

Evaluate the test failures due to differing `_FillValue`s on output and decide whether to:
* re-propose the previous XIOS2 to XIOS3
* propose an alternative feature for XIOS3 to better meet our output missing data requirement

### Further details of the issue.

_No response_

Start by locating the LFRic tests that fail because output _FillValue values differ, then inspect the XIOS3 output path and the referenced XIOS2 commit and nc4_data_output.cpp. Compare the current behavior with XIOS2 and document whether porting that behavior or an alternative meets the missing-data output requirement; the relevant tests should pass with the chosen behavior.

Written by the indexing model from the issue text.