Fixes https://github.com/denoland/deno/issues/26505
I'm not exactly sure how this case comes about (I tried to write tests
for it but couldn't manage to reproduce it), but what happens is the
parent filename ends up null, and we bail out of resolving the specifier
in package exports.
I've checked, and in node the parent filename is also null (so that's
not a bug on our part), but node continues to resolve even in that case.
So this PR should match node's behavior more closely than we currently
do.
