dbconsole: statement details pages should handle empty plan gists gracefully

The plan gist can be empty for queries where the `DisablePlanGist` session var is true. Today DB Console does not handle this in the statement details page, showing a row with an empty plan gist string in the explain tab view. The statement bundle flow also expects a valid plan gist string and crashes when requesting a bundle for said statements.

**Desired**
We should be able to request stmt bundles for stmts without plan gists, and filter out empty plan gist entries from the table.

Jira issue: CRDB-42057