`/keys/upload` called with `device_keys: null`

[richvdh]

Synapse recently added some validation to /keys/upload (element-hq/synapse#17097). As a result of that, it turned out that Element Web was calling /keys/upload with device_keys: null.

Synapse has worked around this for now (element-hq/synapse#19023) but it remains non-spec-compliant behaviour on the client side which we need to fix.

(Seems this is a problem in the request macro in matrix-sdk-crypto-wasm: https://github.com/matrix-org/matrix-sdk-crypto-wasm/blob/ce212dab039ba5758140e0082f39c5260546d2e3/src/requests.rs#L370)

[richvdh]

On closer inspection, turns out we have already been down this path, and fixed it, in Element Web. See #26566 for the original issue, and matrix-org/matrix-sdk-crypto-wasm#72 for the fix.