avoid getting config parser when there is no config to parse

The benefit here is that external special remotes will need a
LISTCONFIGS request and response to generate their config parser,
and this avoids it being done for all the ones that don't have any
configs.

Note that, a config parser could in theory fail to parse if there are no
configs (none currently do), but a parse failure is already thrown away
when generating the remote list because it's too late. Such problems
have to be caught at initremote/enableremote time, not here.

Remote/List.hs

@@ -110,7 +110,9 @@ remoteGen m t g = do
 	let cu = fromMaybe u $ remoteAnnexConfigUUID gc
 	let rs = RemoteStateHandle cu
 	let c = fromMaybe M.empty $ M.lookup cu m
-	pc <- either mempty id . parseRemoteConfig c <$> configParser t
+	pc <- if null c
+		then pure mempty
+		else either mempty id . parseRemoteConfig c <$> configParser t
 	generate t g u pc gc rs >>= \case
 		Nothing -> return Nothing
 		Just r -> Just <$> adjustExportImport (adjustReadOnly (addHooks r)) rs