For self-initializing segments, DefaultTrackOutput receives
a null format and then the actual format. This broke the
deduplication logic in InfoQueue.format, since there were
two transitions: FormatX->Null and Null->FormatX. This CL
allows deduplication to succeed in this case.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129081583
