CLS is not reported on first hidden if the value is still zero

[philipwalton]

Pages with a CLS value of 0 still need to report that value when the page is first hidden. However, the current callback logic won't fire a callback if the delta is 0 and the metric is not final.

This logic should be updated to also check if no previous value exists (to allow for first-time reporting of 0-delta metrics).