Tests: Replace use of `$this->server` with `rest_get_server()` in...

Tests: Replace use of `$this->server` with `rest_get_server()` in `test_get_additional_field_registration_null_schema()`.

In [42724], `$this->server` was replaced with `rest_get_server()` for better memory recycling.

[43908], from the 5.0 branch, was merged into trunk in [44254] and used the now unavailable `$this->server`.

This updates the new test from the 5.0 branch to use the expected `rest_get_server()`.

See #45220, #41641.
Built from https://develop.svn.wordpress.org/trunk@44256


git-svn-id: https://core.svn.wordpress.org/trunk@44086 1a063a9b-81f0-0310-95a4-ce76da25c4cd