drmInitData can be null in DASH if all of the init data is
specified at the manifest level instead. I took a look at
injecting the manifest format into the extractors, so that
we can actually copy the scheme type into it, but that's
at least non-trivial enough to delay for a subsequent CL.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161956246
a2ffcec200