http-api: T5175: check value of attribute for FastAPI>=0.91.0

FastAPI v0.92.0 (starlette v0.24.0) sets default attribute
Request._form; check for value == None instead of existence when
overriding Request class.

(cherry picked from commit cac55a066e1f7d7d4c0eec5ada76570a326b5ac9)