[Beats CM] Beat without tags results in error when retrieving configurations

[exekias]

Testing Kibana 6.5 branch. If I start a beat with no tags assinged I get:

2018-10-26T10:32:15.488+0200	ERROR	[centralmgmt]	management/manager.go:167	error retriving new configurations, will use cached ones: [action_request_validation_exception] Validation Failed: 1: no documents to get;

This should not result in an error, but an empty list of configurations.

This is not a critical bug as it doesn't affect functionality, it's just misleading for the user

@mattapperson @justinkambic

[exekias]

closed by #24665