splitmuxsink: Support NULL muxer

Submitted by Baby octopus

Description

Hi,

I'm figuring a way out to periodically upload thumbnails to an FTP location

.. ! videorate ! video/x-raw,framerate=1/3 ! jpegenc ! curlftpsink

Unfortunately, curlftpsink just creates one big file rather than creating many running files

One way to do this would be through splitmuxsink by having NULL or identity as the muxer. In this way, each buffer would be uploaded to the FTP buffer
This would also need next file indicator i.e., buffer, duration etc which is missing in splitmuxsink