NuPlayer: Only open audio sink if format is valid

In offload use-cases, openAudioSink() is called on every iteration of
kWhatScanSources in case the audio format has changed. Within this call,
createPCMMetaFromSource() copies the required keys from the actual source meta.
convertMetaDataToMessage() is applied to this new result, but it asserts if
specific audio keys are not found. If the source meta is NULL, there are no keys
to propagate to the PCM meta, and this assertion is triggered.

Only call openAudioSink() for offloaded audio if the source format is non-NULL
to avoid this case. Also fix a logging bug & increase the logging level to flag
when a NULL meta is passed to createPCMMetaFromSource().

CRs-Fixed: 799640
Change-Id: Ie87261e3c8f632b2a4f52d0048c3287d83b03ff9