[Connector API] Don't index literal nulls to connector doc

[jedrazb]

Changes

As part of elastic/elasticsearch-specification#2675, we realized that when creating a connector, all properties are written to the index, even if they have a null value. This creates inconsistency between what can be optional and what is actually nullable (semantic-null). We don't need to store literal nulls in the connector index; it's fine to treat connector properties as optionals.

Previously, null values were written to the index to match the status quo (where every single property was preset to null), but this is unnecessary.

This PR ensures that properties are only written to the index upon connector creation if the field is non-null. As a result, fields like last_seen or sync_cursor won't be written to the index and won't be present in the response if they are null. This changes the current behavior where these fields are set to null.

I've tested this change with Kibana, Connector APIs, and the connector service, and everything is working fine. Additionally, this fix will resolve failing client tests due to properties that contain null values.

Code references

The logic to create connector is in the ConnectorIndexService, see createConnector method. This initialises Connector object with user-provided args and presets other properties to default values. There are a number of undefined properties (like last_seen, sync_cursor, etc).

This logic uses Connector's toXContent method to write "JSON" to index. The changes to toXContent from this PR ensure that we don't write nulls to index.

[elasticsearchmachine]

Pinging @elastic/ingestion-team (Team:Search - Extract & Transform)

[elasticsearchmachine]

Pinging @elastic/ent-search-eng (Team:SearchOrg)

[jedrazb]

@elasticsearchmachine merge upstream

[jedrazb]

@elasticmachine merge upstream