REST API: Fix incorrect uses of `rest_sanitize_value_from_schema()`.

In the `check_username()` and `check_password()` callbacks in the Users controller cast the provided request value to a string. The `rest_sanitize_value_from_schema()` function was being used incorrectly which was causing unintended request parsing. 
In `rest_sanitize_request_arg()` do not pass nonexistent third parameter for the `rest_sanitize_value_from_schema()` function.

Props jnylen0, joehoyle, rachelbaker, ocean90.
Fixes #38984.
Built from https://develop.svn.wordpress.org/trunk@39400


git-svn-id: https://core.svn.wordpress.org/trunk@39340 1a063a9b-81f0-0310-95a4-ce76da25c4cd