Fix TypeError on get_attributes when result has no columns

Author: fantix

asyncpg/protocol/prepared_stmt.pyx

@@ -40,6 +40,9 @@ cdef class PreparedStatementState:
     def _get_attributes(self):
         cdef Codec codec
 
+        if not self.row_desc:
+            return ()
+
         result = []
         for d in self.row_desc:
             name = d[0]

tests/test_codecs.py

@@ -1398,3 +1398,7 @@ async def test_enum_and_range(self):
                 DROP TABLE testtab;
                 DROP TYPE enum_t;
             ''')
+
+    async def test_no_result(self):
+        st = await self.con.prepare('rollback')
+        self.assertTupleEqual(st.get_attributes(), ())