User verification doesn't respect that `m.room.power_levels` key `state_default` is optional and defaults to 50

[reivilibre]

prosody-mod-auth-matrix-user-verification/mod_auth_matrix_user_verification.lua

Line 155 in 2839499

if data.power_levels.user >= data.power_levels.room.state_default then

https://spec.matrix.org/v1.8/client-server-api/#mroompower_levels

When state_default is not set, we should assume it is 50.

The Rust SDK currently omits this key when setting new power levels, if it is at its default value.