wmi.getall[] return value in case if empty dataset is returned

Behaviour of wmi.getall[] in agent2 and agentd has discrepancy at this moment. In agent2 wmi.getall[] returns null in case if valid query returned no items.

Steps to reproduce:

1. Create item with key wmi.getall[]. Use a valid WMI query that return no items.
   For example:

   wmi.getall[root\cimv2,"select * from win32_service where displayname like 'non_existing_service'"]

Result:
null returned

Expected:
Return empty JSON array:

[]