Opaque error in stacked bar area when there's no data to plot

[philrz]

Repro is with Zui commit bfbb55e.

When a user clicks the + and selects New Query Session it starts a new tab without a pool pin and hence there's no initial Zed query or results. As shown in the attached video, at this point the stacked bar chart area shows the error message No date range found with 'ts'.

Verify.mp4

As detailed in #3079, this message used to be undefined: pool not found, which was also not ideal.

Since this state came about through an innocent set of clicks in the app, it seems we should catch this case and surface something a bit clearer and phrased less like an error.

[philrz]

Verified in Zui commit 21abbf6.

As shown in the attached video, now when clicking the + and selecting New Query Session, the stacked bar chart is no longer shown. A different message appears below No Pool Specified based on whether or not there's existing pools.

Verify.mp4

Thanks @jameskerr!