/join requests with malformed body

[richvdh]

We see lots of this on requests to POST /_matrix/client/r0/join/{roomIdOrAlias} from clients with user-agents like Riot/1.8.23 (iPhone; iOS 15.5; Scale/2.00):

2022-07-26 00:27:14,236 - synapse.http.servlet - 663 - WARNING - POST-1099409 - Unable to parse JSON: Expecting value: line 1 column 1 (char 0) (b'')

... which suggests it is making requests with a malformed (empty?) body. Currently Synapse accepts this, but it's not spec-compliant and we'll be fixing this in Synapse soon: matrix-org/synapse#13388.

Please could you make sure to include (at least) an empty json object in the request body?