f_decoder_wrapper, sd_add: accept "null" codec

This is for easier use with the "delay_open" feature added in the
previous commit. The "null" codec is reported if the codec is unknown
(because the stream was not opened yet at time the tracks were added).
The rest of the timeline mechanism will set the correct codec at
runtime. But this means every time a delay-loaded track is selected, it
wants to initialize a decoder for the "null" codec.

Accept a "null" decoder. But since FFmpeg has no such codec, and out of
my own laziness, just let it fall back to "common" codecs that need no
other initialization data.

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