Fix empty array literals in SQL adapter (#6806)

Apparently the `elements` field gets omitted from pg_query's output
when there aren't elements.

edb/pgsql/parser/ast_builder.py

@@ -77,7 +77,7 @@ def _list(
     builder: Builder,
     mapper: Callable[[T], U] = _ident,
 ) -> List[U]:
-    return [mapper(builder(n, ctx)) for n in node[name]]
+    return [mapper(builder(n, ctx)) for n in node.get(name, [])]
 
 
 def _maybe_list(

tests/test_sql_query.py

@@ -501,6 +501,13 @@ async def test_sql_query_32(self):
         )
         self.assert_shape(res, 3, ['a'])
 
+        res = await self.scon.fetch(
+            '''
+            SELECT unnest(ARRAY[]::int8[]) a
+            '''
+        )
+        self.assertEqual(len(res), 0)
+
         res = await self.scon.fetch(
             '''
             SELECT *, unnested_b + 1 computed