Don't crash if the MediaCodecList is null.

We need to avoid calling the JavaMediaCodecListWrapper constructor if
the MediaCodecList is null because the constructor assumes its argument
to be non-null. Since throwing a JNI exception does not implicitly
return we need to return null to avoid entering the constructor.

Change-Id: If473d2f6f219719ff81c29307637f28b7e31af77