Description of problem:
Unable to create connection-factory or pooled-connection-factory when the connector property is left unset. 

The error message returned by AS7 after the create child command is submitted:
Null key for a Map not allowed in JSON (use a converting NullKeySerializer?), rolled-back=false, rolled-back=false

The same error message is visible when the connector property is unset on existing subresources.

Paths of affected resources:
*/subsystem=messaging/hornetq-server=default/connection-factory=*
*/subsystem=messaging/hornetq-server=default/pooled-connection-factory=*


How reproducible:
Every time


Steps to Reproduce:

Resource Creation:
1. Discover and import an AS7 standalone server
2. Find a resource hornetq server in the messaging subsystem
3. Create a new connection-factory or a pooled-connection-factory. Leave all the fields to default value. Add values to all required fields that do not have a default value.

Resource Configuration Update:
1. Discovery and import an AS7 standalone server
2. Find resources at the addresses mentioned above.
3. Unset the connector property.

  
Actual results:
The operation fails with the following error message:
Null key for a Map not allowed in JSON (use a converting NullKeySerializer?), rolled-back=false, rolled-back=false

Expected results:
The operation succeeds.


Additional info:
The two test cases fail because of the same issue, attempting to pass a map with a null key to AS7 for the connector field.