fix empty response into webhook (#1665)

CHANGES/1664.bugfix.rst

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+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:
+
+::
+
+   {}