Linux ASAN Fix: NULL reference on OnEncodedVideo.

There are Linux ASAN failure on the below stack.

==938581==ERROR: AddressSanitizer:
==938581==The signal is caused by a READ memory access.
==938581==Hint: address points to the zero page.
SCARINESS: 10 (null-deref)
media::Muxer::VideoParameters const&, scoped_refptr, s

The failure is due to empty content of the encoded_data object.

Bug: 376516525
Change-Id: I59d2814693db7f2f34714d1f8c26834067291ed3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5984025
Reviewed-by: Gabriel Brito <gabrielbrito@microsoft.com>
Commit-Queue: Sunggook Chue <sunggch@microsoft.com>
Reviewed-by: Markus Handell <handellm@google.com>
Cr-Commit-Position: refs/heads/main@{#1377148}