The "combine" function for the int4 sum/avg aggregate functions is
backported to Greenplum 6.0 in commit 313cef6e (from
postgres/postgres@11c8669c0cc) but we made the inevitable omission of
setting the "strictness" of int4_avg_combine to false. This by itself is
harmless as the actual function body *does* guard against NULL input,
but it also prohibits a whole host of optimizations when the executor
and planner can detect NULL input early on and short-circuit the
execution. Oops.

This patch flips the `proisstrict` flag back to true for
int4_avg_combine.

Backpatch to 6X_STABLE.

(cherry picked from commit c334bedc)