video: add a default color space for files with no video

Usually mp_image_params_guess_csp takes care of finding *some* default
for a video channel, but files with no video (or with extremely broken
configurations) end up leaving the colorspace information as
MP_CSP_PRIM_AUTO, which has no associated primaries.

As a result of this, color managed OSD messages could not display
because they were being color managed to match the (non-existing/absurd)
video channel. With this change, such non-spaces will have BT.709
primaries as far as color management and the OSD is concerned.

This fixes #961.

CC: @mpv-player/stable

Signed-off-by: wm4 <wm4@nowhere>