fix: no need to allow_nil_input for an unaccepted field

the purpose of `allow_nil_input` is to say "we accept some required
field X as input, but the action will set it if the user does not".
Non-accepted attributes *always* behave that way, expecting the
action to set them, and are checked just prior to committing the action.

lib/ash_authentication/strategies/password/transformer.ex

@@ -193,7 +193,6 @@ defmodule AshAuthentication.Strategy.Password.Transformer do
       arguments: arguments,
       changes: changes,
       metadata: metadata,
-      allow_nil_input: [strategy.hashed_password_field],
       description: "Register a new user with a username and password."
     )
   end