n-api: guard against cond null dereference

A condition variable is only created by the thread-safe function if the
queue size is set to something larger than zero. This adds null-checks
around the condition variable and tests for the case where the queue
size is zero.

Fixes: https://github.com/nodejs/help/issues/1387
PR-URL: https://github.com/nodejs/node/pull/21871


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>