Fix ? operator crash on an agtype null left operand (#2589)

For a scalar left operand, agtype_exists_agtype() passes the result of
extract_entity_properties() straight to agtype_value_to_agtype(). For an
agtype null (or an entity whose properties are null) that result is NULL,
so the backend dereferences a NULL pointer and the postmaster restarts:

    SELECT 'null'::agtype ? '"a"'::agtype;

Return false when there are no properties to search, the same answer the
operator already gives for other non-container scalars such as '1' or
'"s"'. Add regression cases to jsonb_operators.

Fixes 2562

Author: akashchamp

regress/expected/jsonb_operators.out

@@ -189,6 +189,18 @@ SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::agtype ? '["e1"
  f
 (1 row)
 
+SELECT 'null'::agtype ? '"a"'::agtype;
+ ?column? 
+----------
+ f
+(1 row)
+
+SELECT 'null'::agtype ? 'null'::agtype;
+ ?column? 
+----------
+ f
+(1 row)
+
 -- errors out
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::agtype ? 'e1';
 ERROR:  invalid input syntax for type agtype

regress/sql/jsonb_operators.sql

@@ -60,6 +60,8 @@ SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::agtype ? '["n",
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::agtype ? '{"n": null}';
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::agtype ? '{"n": null, "b": true}';
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::agtype ? '["e1"]';
+SELECT 'null'::agtype ? '"a"'::agtype;
+SELECT 'null'::agtype ? 'null'::agtype;
 
 -- errors out
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::agtype ? 'e1';

src/backend/utils/adt/agtype_ops.c

@@ -1322,7 +1322,15 @@ Datum agtype_exists_agtype(PG_FUNCTION_ARGS)
 
     if (AGT_ROOT_IS_SCALAR(agt))
     {
-        agt = agtype_value_to_agtype(extract_entity_properties(agt, false));
+        agtype_value *properties = extract_entity_properties(agt, false);
+
+        /* an agtype null (or null properties) has no keys to look up */
+        if (properties == NULL)
+        {
+            PG_RETURN_BOOL(false);
+        }
+
+        agt = agtype_value_to_agtype(properties);
     }
 
     if (AGT_ROOT_IS_SCALAR(key))