parsebin: Fix assertions/regression when dealing with un-handled streams

playbin3: Don't use unknown types for default selection

When creating a fallback default selection from a collection, don't attempt to
use unknown stream types

parsebin: Don't modify inexistant GstStream

When handling exposing un-handled streams, we can only replace the GstStream for
those we are creating ourselves (i.e. the fallback collection).

Fixes assertions when the demuxer creates those streams