`query_partition_range_concurrent` implements an optimization when
querying a token range that intersects multiple vnodes. Instead of
sending a query for each vnode separately, it sometimes sends a single
query to cover multiple vnodes - if the intersection of replica sets for
those vnodes is large enough to satisfy the CL and good enough in terms
of the heat metric. To check the latter condition, the code would take
the smallest heat metric of the intersected replica set and compare them
to smallest heat metrics of replica sets calculated separately for each
vnode.
Unfortunately, there was an edge case that the code didn't handle: the
intersected replica set might be empty and the code would access an
empty range.
This was catched by an assertion added in
8db1d75c6c by the dtest
`test_query_dc_with_rf_0_does_not_crash_db`.
The fix is simple: check if the intersected set is empty - if so, don't
calculate the heat metrics because we can decide early that the
optimization doesn't apply.
Also change the `assert` to `on_internal_error`.
Fixes#14284Closes#14300
732feca115