bug: SupabaseAdapter is missing the onConflict override

[devj3ns]

My Customer model has no onConflict specified in the SupabaseSerializable, which results in the following runtime error:

lib/brick/adapters/customer_adapter.g.dart:116:7: Error: The non-abstract class 'CustomerAdapter' is missing implementations for these members:
 - SupabaseAdapter.onConflict

Current behavior: When onConflict is not specified in the SupabaseSerializable, the generator does not override the onConflict field in the OfflineFirstWithSupabaseAdapter.

Expected behavior: The generator overrides the field as follows, when onConflict is not specified:

@override
final onConflict = null;

related: #422

[tshedor]

Fixed and published in #430