Summary:
In `indxpath.c` there is an overly strict assert added by D34809/24433598420516cde9208c3fc60d22d83917de77 that checks to see if either there are no unbatchable clauses or all index key columns involve unbatchable clauses. It could be possible as in this test case that only a strict subset of the key columns could involve unbatchable clauses.

This fixes a bug that first shows up in 2.20 on debug builds.
Jira: DB-11819

Test Plan: ./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressJoin'

Reviewers: mtakahara

Reviewed By: mtakahara

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D38699