Summary:
Till we add support for NULLS NOT DISTINCT (GH #24399), block the command.
Jira: DB-13358

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

Reviewers: fizaa

Reviewed By: fizaa

Subscribers: smishra, yql

Differential Revision: https://phorge.dev.yugabyte.com/D38969