Fix of the get pushers method usage with synapse

[predictron-cloud]

In Synapse for the new users the pushers are created with lang == null, but in the dart sdk it was required field.

https://github.com/matrix-org/synapse/blob/3de82bb2af28f56696a79bf41ccffc81385b6e2c/synapse/handlers/register.py#L1022

https://github.com/matrix-org/synapse/blob/3de82bb2af28f56696a79bf41ccffc81385b6e2c/synapse/rest/admin/users.py#L470

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[nico-famedly]

Hm, this field is listed as required in the specification and we just generate the models directly from the openapi spec. As such the proper fix for this would be to file a spec issue and in the meantime add a patch to our existing patch set to not mark this field as required and regenerate the spec.

Upstream requirement: https://github.com/matrix-org/matrix-spec/blob/99e2ff4927b56db927d52e82ff802b38f6fa8407/data/api/client-server/pusher.yaml#L103
Our patches: https://github.com/famedly/dart_openapi_codegen/tree/main/scripts

So while your change would work temporarily, it would break with the next spec update, if it isn't added as a patch.

[nico-famedly]

I filed a spec issue for you, feel free to add context there, if you have any: matrix-org/matrix-spec#1643