improvement(RPC): make params payload flexible for certain RPCs

[onur-ozkan]

Makes get_enabled_coins, task::init_trezor::init and wc_get_sessions RPCs to accept empty params field without any breaking change on the API.

Closes #2498

[onur-ozkan]

cc @smk762

[smk762]

Thanks - I've confirmed this is working for v2 get_enabled_coins, but the other two methods (task::init_trezor::init, wc_get_sessions) in the linked issue still return the same error response, and in logs:

04 08:41:55, mm2_main::rpc:177] ERROR RPC dispatcher error: dispatcher:144] Error parsing request: invalid type: null, expected struct RpcInitReq
04 08:42:19, mm2_main::rpc:177] ERROR RPC dispatcher error: dispatcher:144] Error parsing request: invalid type: null, expected struct EmptyRpcRequest

[onur-ozkan]

7923bf9 should fix them.

[smk762]

Confirmed params presence is permissive on all 3 methods listed in issue. Thanks!