Fix AttributeError when executing empty or comment-only scripts

When executing a script that contains only comments or is empty,
PostgreSQL returns no result status message. This caused an
AttributeError when trying to decode None.

Fixes #1259

Author: veeceey

asyncpg/protocol/protocol.pyx

@@ -801,7 +801,10 @@ cdef class BaseProtocol(CoreProtocol):
         waiter.set_result(self.result)
 
     cdef _on_result__simple_query(self, object waiter):
-        waiter.set_result(self.result_status_msg.decode(self.encoding))
+        if self.result_status_msg is not None:
+            waiter.set_result(self.result_status_msg.decode(self.encoding))
+        else:
+            waiter.set_result(None)
 
     cdef _on_result__copy_out(self, object waiter):
         cdef bint copy_done = self.state == PROTOCOL_COPY_OUT_DONE

tests/test_execute.py

@@ -63,6 +63,19 @@ async def test_execute_script_check_transactionality(self):
                 SELECT * FROM mytab
             ''')
 
+    async def test_execute_empty_script(self):
+        # Test executing a script with only comments (issue #1259)
+        result = await self.con.execute('-- hello')
+        self.assertIsNone(result)
+
+        # Test executing a script with only whitespace
+        result = await self.con.execute('   ')
+        self.assertIsNone(result)
+
+        # Test executing an empty script
+        result = await self.con.execute('')
+        self.assertIsNone(result)
+
     async def test_execute_exceptions_1(self):
         with self.assertRaisesRegex(asyncpg.PostgresError,
                                     'relation "__dne__" does not exist'):