Don't set custom profile fields to null when deletion is meant.

[reivilibre]

Null is not interoperable as an allowable field value:

Servers MAY reject null values.
— https://spec.matrix.org/v1.19/client-server-api/#put_matrixclientv3profileuseridkeyname

Synapse will enforce this to help in element-hq/synapse#20153

Complement updated in matrix-org/complement#914

Checklist

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[CLAassistant]

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[reivilibre]

I think this flaked? Please do ping me if that's not the case

[florianduros]

@reivilibre Tests are not flaking, a snapshot needs to be updated: https://github.com/element-hq/element-web/actions/runs/32991277157/job/98249165177?pr=34844

[reivilibre]

@florianduros I don't get that, as I haven't changed any user-visible markup or anything like that. Any ideas?

[florianduros]

Indeed, it was a flake mb