Non required field in connector config

[l-trotta]

As explained in the latest connector PR, some of the ConnectorConfig properties are only nullable to support an edge case which will be removed from the server. In the meantime though, this breaks the java client, so for now let's make them nullable.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
connector.check_in	🟢	3/3	3/3
connector.delete	🟢	9/9	9/9
connector.get	🔴	60/60	59/60
connector.last_sync	🟢	7/7	7/7
connector.list	🟢	19/19	19/19
connector.post	🟢	7/7	7/7
connector.put	🟢	13/13	13/13
connector.secret_delete	🟠	Missing type	Missing type
connector.secret_get	🟠	Missing type	Missing type
connector.secret_post	🟠	Missing type	Missing type
connector.secret_put	🟠	Missing type	Missing type
connector.sync_job_cancel	🟢	3/3	3/3
connector.sync_job_check_in	🟠	Missing type	Missing type
connector.sync_job_claim	🟠	Missing type	Missing type
connector.sync_job_delete	🟢	4/4	4/4
connector.sync_job_error	🟠	Missing type	Missing type
connector.sync_job_get	🟢	22/22	22/22
connector.sync_job_list	🟢	12/12	12/12
connector.sync_job_post	🟢	50/50	50/50
connector.sync_job_update_stats	🟠	Missing type	Missing type
connector.update_active_filtering	🟢	1/1	1/1
connector.update_api_key_id	🟢	4/4	4/4
connector.update_configuration	🔴	8/9	9/9
connector.update_error	🟢	4/4	4/4
connector.update_features	🟠	Missing type	Missing type
connector.update_filtering_validation	🟢	3/3	3/3
connector.update_filtering	🟢	12/12	12/12
connector.update_index_name	🟢	4/4	4/4
connector.update_name	🟢	4/4	4/4
connector.update_native	🟢	3/3	3/3
connector.update_pipeline	🟢	3/3	3/3
connector.update_scheduling	🟢	3/3	3/3
connector.update_service_type	🟢	2/2	2/2
connector.update_status	🟢	3/3	3/3

You can validate these APIs yourself by using the make validate target.