src: delete BaseObjectWeakPtr data when pointee is gone

Fix the condition for deleting the underlying data pointed to by
a `BaseObjectWeakPtr`, which erroneously skipped that deletion
when `ptr->get()` was `nullptr`. This fixes a memory leak reported
by some of the tests.

Refs: https://github.com/nodejs/node/pull/30374#issuecomment-601848973

PR-URL: https://github.com/nodejs/node/pull/32393


Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>