ui: Do not graph series which have no data points

[mrtracy]

Graph lines which have no data points in the queried time span will no
longer be displayed at all; they will not appear in the legend or the
interactive tooltip. This situation has proved a consistent source of
confusion, especially on graphs which display (empty) values for
decommissioned nodes which are no longer producing data.

Fixes #19541

Release note: Fixed an issue where graph tooltips would continue to
display zero values for nodes which had long been decommissioned.

[cockroach-teamcity]

This change is 

[couchand]

code LGTM. My only question is if we want to check that the node is decommissioned first, but I guess perhaps that would be better to roll into the wider question of how to show "no data" points for non-decomissioned nodes.

This still doesn't close the door on UX for decomissioned nodes, for instance #19782 was supposed to be related to decomissioning, and I wouldn't be surprised if other surprises crop up, but I suppose this makes progress.

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[mrtracy]

This isn't meant to close the door on decommissioned nodes, the issue it is resolving is very specific (nodes polluting graph tooltips); this is an issue that users are encountering and complaining about.

The "corollary", if you will - a node which is live according to other metrics, but is not recording any time series when expected - doesn't seem to be a likely enough scenario that we need to tackle it in the same swipe.

[mrtracy]

After discussing this some more with @couchand, we agree that it's better to merge this than not. We think that there might be some cases where the absence of data for a line needs to be conspicuous, but they are outweighed by the annoying cases where it should be hidden, and will ultimately be better addressed by events-on-graphs or a similar feature.