Improve default explorer empty state message

[amlannandy]

This is the empty state we currently show in the Explorer section when

1. We initially land on the explorer and no query has been run
2. When we run a query, but there's no data for it.

We want to improve the error messages for both the cases here:

[request-info]

We would appreciate it if you could provide us with more info about this issue/pr!