test: add n-api null checks for conversions

Add assertions that conversion and coercion N-APIs return appropriate
error statuses when given `NULL`s for parameters they expect to not be
`NULL`.

For `napi_get_value_string_*` this also checks that it returns
`napi_string_expected` when passed a `napi_value` not containing a
string.

PR-URL: https://github.com/nodejs/node/pull/34142


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>