Suppress empty body warnings in room servelets (#14600)

* Suppress empty body warnings in room servelets

We've already decided to allow empty bodies for backwards compat. The
change here stops us from emitting a misleading warning; see also
https://github.com/matrix-org/synapse/issues/14478#issuecomment-1319157105

* Changelog

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/14600.bugfix b/changelog.d/14600.bugfix
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+Suppress a spurious warning when `POST /rooms/<room_id>/<membership>/`, `POST /join/<room_id_or_alias`, or the unspecced `PUT /join/<room_id_or_alias>/<txn_id>` receive an empty HTTP request body.