Add `@Nullable` to changePassword parameters in `UserDetailsManager`

[therepanic]

Also, an unused argument was removed from JdbcUserDetailsManager#createNewAuthentication that was not used and would break the build of the current change.

Closes: gh-18257

[therepanic]

Since there's a test that checks protected JdbcUserDetailsManager#createNewAuthentication with a password (currently a pointless test), I decided to annotate it with @Nullable instead of deleting it.

On the other hand, it might be better to delete the test, since again, the argument isn't used and it's a protected method. WDYT?