fix: omit null table from postgres_changes join payload

[Tr00d]

What

A schema-wide postgres_changes listener has no table, so Table is null.
That was serialized into the join config.postgres_changes payload as
"table": null, whereas the server expects the key to be absent. Adding
NullValueHandling.Ignore drops the key when Table is null.

// before — schema-wide listener
{ "event": "*", "schema": "public", "table": null }
// after
{ "event": "*", "schema": "public" }

Why

Sending "table": null is not the shape the realtime server expects for a
schema-wide subscription; omitting the key matches the reference realtime-js
config object, where an absent table means "every table in the schema."

Wire-shape change

This changes the serialized join payload for schema-wide listeners only
("table": null → key omitted). Listeners that specify a table are unaffected.
Called out here for reviewers; the package has no payload snapshot suite, so
there is nothing to re-approve.

Relationship to #54

Third of three focused PRs carved out of the stale draft #54, rebased onto
current master and brought to standard. #54 bundled this attribute change
with an unrelated [Obsolete] on PostgresChangesOptions.Parameters; that
deprecation is intentionally left out of this PR and deferred to the follow-up
that retires the two-step postgres_changes API.

Testing

• PostgresChangesOptionsTests (hermetic serialization): table is omitted
  when null, and serialized when provided. Verified the null case catches the
  regression — pre-fix it fails with the table key present.
• Full realtime suite green (45/45). Clean build, zero new warnings.