OxQL metrics: don't cut off the part of the time range with no data

I started an instance and messed with it a bit to test the charts. I made the time range 3 hours, including time before the instance existed and in the future. The time before comes back with no data, while the time in the future comes back with nulls. We treat these differently: we display the nulls but not the nothing.

I'm not 100% sure whether we should always display the full range asked for — on one hand I was pretty confused about why my graph started at a time other than the one I put in the picker. On the other hand, if you ask for a big range and there's only data for a little bit of it, you get a pretty useless graph. On the third and hopefully final hand, in that situation the user can simply change the time range to zoom in on the relevant slice. (This rules out bad ideas I had like a checkbox about whether to show the full range or only spots where we have actual data. That's hard to explain and, more importantly, covered by the existing controls.)

Reproduce the OxQL metrics chart with a range spanning before instance creation and into the future, then trace how empty results and nulls are handled. No files, tests, or entry points are named, and the expected full-range behavior remains undecided; confirm that decision before defining done.