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/backend/parser/gram.y

@@ -7990,7 +7990,11 @@ opt_routine_body:
 routine_body_stmt_list:
 			routine_body_stmt_list routine_body_stmt ';'
 				{
+					/* As in stmtmulti, discard empty statements */
+					if ($2 != NULL)
 						$$ = lappend($1, $2);
+					else
+						$$ = $1;
 				}
 			| /*EMPTY*/
 				{

src/test/regress/expected/create_function_3.out

@@ -267,7 +267,7 @@ 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
     LANGUAGE SQL

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