heic content_encoding field is optional

comply with the ISO spec that allows for content type fields to
be omitted in the mp4 header if they would be otherwise empty.

Bug: 74375003
Test: before/after of PoC
Change-Id: I522bacf192fee86122f603d6e87506e1a004f3f6
(cherry picked from commit 237dc91c546cf5e39737812f49460b552a529f7d)
1 file changed