Minor: add an explicit indicator that null packets were removed

[t0ms]

Context: one of a set of separate follow-ups to #7, from evaluating m2ts packaging for broadcast primary distribution / contribution handed off to hardware IRDs (SRT/Zixi/RIST-style satellite replacement). Background: https://github.com/tdrapier-wbd/moq-mpegts-paper (public technical evaluation, not yet widely announced). This is a minor convenience item, related to the m2tsMuxRate issue.

Problem

Multi-Program Source Handling ({#mpts}) lets null packets (PID 0x1FFF) "be removed or retained at the publisher's discretion," and pairs removal with the advisory m2tsMuxRate. But there is no explicit signal of whether removal actually happened:

• m2tsMuxRate presence is only a soft proxy — it might be present purely for information even when nulls are retained; and
• otherwise a subscriber has to inspect the stream to tell.

Why it helps contribution → IRD

An egress that must decide whether to re-stuff back to CBR benefits from an unambiguous declaration up front. (A receiver can also detect 0x1FFF presence by inspection, so this is a convenience/advisory hint, not a correctness requirement.)

Suggestion

Add an optional boolean (e.g. m2tsNullsRemoved) indicating that PID 0x1FFF stuffing was stripped before packaging. It could be documented alongside m2tsMuxRate.