Depends on: https://github.com/matrix-org/complement/pull/914
`displayname` and `avatar_url` already did not permit nulls, seemingly
from #8223.
The specification at v1.15 does not permit `null` for those fields
either, so this seems consistent with historical specification.
This only changes behaviour for 'custom' fields.
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Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>