grpc-js: Fix address equality check in pick-first

[murgatroid99]

This fixes #2379. In the current code, if the pick-first LB policy gets an address list that is shorter than the previous address list, the loop checking for equality can run off the edge of the list and pass an undefined value the type checker doesn't account for into subchannelAddressEqual. I added three layers of protection to avoid this:

1. The loop is skipped if the lengths are equal.
2. subchannelAddressEqual is only called if the value is not undefined.
3. subchannelAddressEqual can now handle undefined values, in case something like this happens elsewhere.