Make MessageSerializer.text field required
Make MessageSerializer.text field required

As documented in [[https://core.telegram.org/bots/api#message]], `Message.text` is optional. When we get an update without `text` in its `message`, `KeyError` is raised because of depending on `validated_data['message']['text']`. By making it required, updates without `text` field will get 400-status response instead of 500.

jlmadurga/django-telegram-bot,jlmadurga/django-telegram-bot