OxQL metrics: temporarily work around `input tables to group_by must be aligned`

[david-crespo]

@bnaecker says we might not have a fix right away for oxidecomputer/omicron#7715 and it's pretty easy to work around. All we have to do is look for the "input tables to \group_by` must be aligned"` error message and treat that as an empty result. We know our query as aligned, so we know that error can only be due to the timeseries being empty.