Always fetch column info from the server, even if there are no rows.

web/pgadmin/utils/driver/psycopg2/__init__.py

@@ -718,16 +718,17 @@ Polling result for (Query-id: {query_id})""".format(query_id=self.__async_query_
 
         status = self._wait_timeout(self.conn, ASYNC_WAIT_TIMEOUT)
         if status == self.ASYNC_OK:
-            if cur.rowcount > 0:
             # Fetch the column information
             colinfo = [desc for desc in cur.description]
+
+            if cur.rowcount > 0:
                 result = []
                 # Fetch the data rows.
                 for row in cur:
                     result.append(dict(row))
                 self.__async_cursor = None
                 return status, result, colinfo
-        return status, None, None
+        return status, None, colinfo
 
     def cancel_transaction(self, conn_id, did=None):
         """