The Matrix Spec requires that requests to POST /rooms/{roomId}/receipt/{receiptType}/{eventId} have a body, even if it's just {}. Unfortunately, older Element Android clients omitted this and sent empty bodies instead.

We worked around this in #10531, but we'd like to eventually remove the workaround per #10534.

To prevent the problem from getting worse, we should limit the scope of the workaround to only apply to known bad clients.

Specifically, instead of unconditionally setting allow_empty_body=True here:

We should only set it to True if the result of get_request_user_agent(request) contains Android and satisfies one of the following:

• Contains Riot (e.g., Old Riot.im, Riot, RiotX, Element (Riot.im))
• Starts with Element/1.[012].*
• Starts with SchildiChat/1.[012].*

(This will unnecessarily allow all Element 1.2.x releases to send empty bodies, instead of just versions < 1.2.1, but doing so really simplifies the glob, and we know that 1.2.1 and later are well behaved, so no harm done.)