grpc-js-xds: Fix a couple of bugs with the config tears change

[murgatroid99]

This change fixes two significant bugs introduced in #2844:

1. pick_first reported IDLE instead of TRANSIENT_FAILURE when provided a zero-length address list. This caused all calls to queue for LB picks instead of ending with an error, which means that they piled up quickly.
2. As a consequence of the way in-use clusters were retained, the xDS dependency manager would provide the config again every time a call was committed (or at least most of the time).