fix(api): let optional fields be cleared on member, group and system edits

[SiteRelEnby]

Found by auditing for the failure mode behind #68. It was not a one-off.

What was broken

Emptying an optional field and saving did nothing - the old value came back.

where	fields
Member	display name, pronouns, colour, birthday, description
Group	description, colour
System profile	description, tag, colour
Both	removing an avatar or banner

Pronouns is probably the one people noticed; avatar removal is the one that looks most like the app ignoring you.

Why

These endpoints read the body with exclude_unset, so presence is the contract:

• omitted -> leave it alone
• JSON null -> clear it
• a value -> set it

Moshi omits null fields, which expresses the first two-thirds and leaves no way to clear anything. The ViewModels already produce null for an emptied field; it just never reached the wire.

Probed rather than assumed - a member edit with five cleared fields serialised to:

{"name":"Alex"}

The server then correctly left everything alone.

Why not just turn on null serialisation

That was the tempting one-liner, and it would have been worse. Each of these bodies also carries fields backed by NOT NULL columns that the server explicitly rejects an explicit null for - a member's name and privacy, a system's date_format, privacy, show_member_created_date and the front defaults. Blanket nulls would start sending those and fail the entire save rather than one field.

So each field is either clearable or omit-when-null, matching the server's _reject_explicit_null lists. The two helpers are named clears(...) and omitsWhenNull(...) so a field added later has to make the choice rather than inheriting a default.

No ViewModel changes

The call sites already do takeIf { it.isNotBlank() }, producing null for an emptied field. That now means what it always should have.

Follow-up when #69 lands

#69 adds emoji to MemberUpdate and works around this by sending an empty string. Once both are in, add clears("emoji", ...) to the member adapter and simplify that workaround to a plain null. There's a comment at the spot so it isn't lost. Nothing breaks in either merge order - the workaround keeps working regardless.

Testing

11 unit tests, built through the app's own Moshi via NetworkModule.provideMoshi() so a missing registration fails here rather than only in the field. They pin both directions: fields that must clear, and fields that must stay absent because a null would fail the save.

:app:assemblePlayRelease, :app:assembleOpenRelease, :app:testPlayReleaseUnitTest green.

Device checklist:

• Member: set pronouns, save, reopen, clear them, save - they stay gone
• Same for display name, birthday, description, colour
• Remove a member's avatar, and a member's banner: both go and stay gone
• Group: clear description and colour
• Settings > Profile: clear the system tag and description, remove the system avatar
• A normal edit that changes one field leaves the others alone
• Renaming a member still works (the NOT NULL path)