Fix assert in BRIN build_distances

Fix assert in BRIN build_distances

When brin_minmax_multi_union merges summaries, we may end up with just a
single range after merge_overlapping_ranges. The summaries may contain
just one range each, and they may overlap (or be exactly the same).

With a single range there's no distance to calculate, but we happen to
call build_distances anyway - which is fine, we don't calculate the
distance in this case, except that with asserts this failed due to a
check there are at least two ranges.

The assert is unnecessarily strict, so relax it a bit and bail out if
there's just a single range. The relaxed assert would be enough, but
this way we don't allocate unnecessary memory for distance.

Backpatch to 14, where minmax-multi opclasses were introduced.

Reported-by: Jaime Casanova
Backpatch-through: 14
Discussion: https://postgr.es/m/YzVA55qS0hgz8P3r@ahch-to