ParsePdu admin command can cause a panic

Reported by M0dex

panic message:

thread 'rocket-worker-thread' panicked at 'ruma can calculate reference hashes: Json(JsonFieldMissingFromObject("type"))', src/database/admin.rs:354:34
stack backtrace:
0: rust_begin_unwind
at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/std/src/panicking.rs:498:5
1: core::panicking::panic_fmt
at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/core/src/panicking.rs:107:14
2: core::result::unwrap_failed
at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/core/src/result.rs:1613:5
3: conduit::database::admin::process_admin_message
4: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
5: tokio::runtime::task::core::CoreStage<T>::poll
6: tokio::runtime::task::harness::Harness<T,S>::poll
7: std::thread::local::LocalKey<T>::with
8: tokio::runtime::thread_pool::worker::Context::run_task
9: tokio::runtime::thread_pool::worker::Context::run
10: tokio::macros::scoped_tls::ScopedKey<T>::set
11: tokio::runtime::thread_pool::worker::run
12: tokio::runtime::task::harness::Harness<T,S>::poll
13: tokio::runtime::blocking::pool::Inner::run