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

src/test/regress/sql/create_function_3.sql

@@ -165,7 +165,7 @@ CREATE FUNCTION functest_S_3() RETURNS boolean
     RETURN false;
 CREATE FUNCTION functest_S_3a() RETURNS boolean
     BEGIN ATOMIC
-        RETURN false;
+        ;;RETURN false;;
     END;
 
 CREATE FUNCTION functest_S_10(a text, b date) RETURNS boolean