Prevent NaN lonShift from crashing hover

[platinops]

Prevent NaN lonShift from crashing hover on maps when xval is undefined.
Fixes #7702.
I created this since I saw PR #7709 was abandoned/deleted by its author.

[camdecoster]

Thanks for the PR. We'll take a look at follow up.

[camdecoster]

I think that just catching NaN values is a band-aid fix. I looked into why the values are NaN to begin with, and it has to do with the fact that xval and yval aren't getting set when Plotly.Fx.hover is called with pointNumber in the payload. Could you see if there's a way to set xval and yval to the proper value? Additionally, there might be other places where these values need to be set.