decodebin3: Always ensure we end up with parsebin or identity

This fixes a regression introduced by 6c4f52ea

There are cases where the input stream will be push-based, time-segment and not have a collection nor any caps. This means the event-based checks are not sufficient to decide when/where to plug in a identity or parsebin to process the input.

For those corner cases we setup a buffer probe to ensure we always end up with at least a parsebin

Fixes #3609 (closed)