fix(realtime): handle null values in postgres changes filter comparison

[7ttp]

summary

fixes the mismatch between server and client bindings for postgres changes error that occurs when subscribing to postgres_changes.

problem

when the client subscribes without a filter:

.on('postgres_changes', {
  event: '*',
  schema: 'public',
  table: 'notifications',
}, callback)

the server returns ilter: null in the response, but the client has ilter: undefined. the strict equality check ull === undefined returns false, causing a false mismatch error.

solution

added a helper method isFilterValueEqual that normalizes null to undefined before comparison. this handles the json serialization discrepancy between server and client.

changes

• RealtimeChannel.ts: use normalized comparison for schema, table, and filter fields
• RealtimeChannel.postgres.test.ts: added test cases for null and omitted filter scenarios

testing

all 21 postgres tests pass including 2 new tests that specifically cover this bug.

closes #1917

cc @mandarini

[coveralls]

coverage: 95.367% (+14.2%) from 81.176%
when pulling 1a5234e on 7ttp:bug1
into 336e498 on supabase:master.