CWLI query in panel should support not selecting log groups

[ruiyewan]

Is your feature request related to a problem? Please describe.
We have a use case where we are building a panel for CloudWatch traces, the data source is a AWS monitoring account CW data source. The monitoring account contains 52 (and growing) log groups that we want to search against, they are all named aws/spans from various source accounts.

The query would fail with 500 error because CW StartQuery has an upper limit of 50 log groups in query. However CW does support more than 50 log groups in a query via SOURCE syntax to match log groups with prefix. If I use SOURCE syntax in the query and unselect all log groups, the query won't execute because the plugin thinks it is an invalid use case

Describe the solution you'd like
The plugin should allow SOURCE syntax in the query with zero selected log groups.

Describe alternatives you've considered

1. A better way could be in the UI the plugin supports matching log groups with prefix (currently it can be done via creating a variable though), and internally converts it to SOURCE syntax then call CW API.
2. Or for queries with a large number of log groups, the plug should break it into multiple smaller queries and aggregate the result

Additional context
Add any other context or screenshots about the feature request here.