Ideally they are already filtered on the embedder side. Sometimes
howevever, embedders end up with a Local<T> pointing to a nullptr
object. In this case the best way to filter this is right at the
beginning of the registration process.

BUG=chromium:713667

Review-Url: https://codereview.chromium.org/2836013003
Cr-Commit-Position: refs/heads/master@{#44844}