Single component representation with DRM issue

[maxbykov]

Hi again,
I've got a feeling that this commit 50a43ff broke single representation (audio only/video only) streams.
Sample manifest: https://gist.github.com/maxbykov/9ab6b4cf73ab537ad4b5bdecb4a420b8
The issue might be here where if there is no video or audio, corresponding
*Capabilities would be undefinedand it will result in DRM.FAILED_TO_CREATE_CDM (Cannot read property 'contentType' of undefined)

[tdrews]

Thanks for the report. We'll take a look.