mpegts: Handle "empty" PMT gracefully
Some streams have 2 PMT sections in a single TS packet. The first one is "valid" but doesn't contain/define any streams. That causes an unrecoverable issue when we try to activate the 2nd (valid) PMT.
Instead of doing that, pre-emptively refuse to process PMT without any streams present within. We still do post that section on the bus to inform applications.
Fixes #1181 (closed)