improve no data display for Top SQL

[breezewish]

Feature Request

Is your feature request related to a problem? Please describe:

Now Top SQL simply displays "No data to display" when there is no data.

Describe the feature you'd like:

Supply more information about why there is no data. For example, when Top SQL is just enabled, the data can be unavailable. In such case, user need to wait. Also, users may select a range without data. In such case, user need to change time range.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[breezewish]

There is an successful attempt to keep axis showing in https://github.com/pingcap/tidb-dashboard/pull/1241

[shhdgit]

Cool! Let me try.