src: add napi_define_class() null checks

napi_define_class is tested by passing NULL to all parameters that are
pointers, one at a time. Moreover, two bugs were corrected. One was
utf8name and the second was the property descriptor pointer. These
pointers were assumed to be non-NULL and now we have NULL checks.

PR-URL: https://github.com/nodejs/node/pull/27945


Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>