pgsql: Fix estimate_array_length error with set-operation array coercio

Fix estimate_array_length error with set-operation array coercions

When a nested set operation's output type doesn't match the parent's
expected type, recurse_set_operations builds a projection target list
using generate_setop_tlist with varno 0. If the required type
coercion involves an ArrayCoerceExpr, estimate_array_length could be
called on such a Var, and would pass it to examine_variable, which
errors in find_base_rel because varno 0 has no valid relation entry.

Fix by skipping the statistics lookup for Vars with varno 0.

Bug introduced by commit 9391f7152. Back-patch to v17, where
estimate_array_length was taught to use statistics.

Reported-by: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Author: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/adjW8rfPDkplC7lF@pryzbyj2023
Backpatch-through: 17

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/13e20d1c9d99516d13f3ee0dc164168a74cde0df

Modified Files
--------------
src/backend/utils/adt/selfuncs.c | 12 ++++++++++++
src/test/regress/expected/union.out | 17 +++++++++++++++++
src/test/regress/sql/union.sql | 4 ++++
3 files changed, 33 insertions(+)