pgsql: Don't crash on empty statements in SQL-standard function bodies.

Don't crash on empty statements in SQL-standard function bodies.

gram.y should discard NULL pointers (empty statements) when
assembling a routine_body_stmt_list, as it does for other
sorts of statement lists.

Julien Rouhaud and Tom Lane, per report from Noah Misch.

Discussion: https://postgr.es/m/20210606044418.GA297923@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bfeede9fa464ab707eb5ac5704742bf78bd7ac9e

Modified Files
--------------
src/backend/parser/gram.y                       | 6 +++++-
src/test/regress/expected/create_function_3.out | 2 +-
src/test/regress/sql/create_function_3.sql      | 2 +-
3 files changed, 7 insertions(+), 3 deletions(-)