Fix empty response into webhook.

We need to return something “empty”, and “empty” form doesn’t work since
it’s sending only “end” boundary w/o “start”.

An empty formdata should look smth like this for Telegram to understand:

::

   --webhookBoundaryvsF_aMHhspPjfOq7O0JNRg
   --webhookBoundaryvsF_aMHhspPjfOq7O0JNRg--

But aiohttp sends only the ending boundary:

::

   --webhookBoundaryvsF_aMHhspPjfOq7O0JNRg--

Such response doesn't suit Telegram servers.

The fix replaces empty response with empty JSON response:

::

   {}