swapchain: don't send invalid HDR swapchain hints

In the absence of metadata fields, we absolutely should infer them to the same values that will be used later on in the pipeline. Otherwise, there will be massive confusion as a result of e.g. libplacebo assuming 0.005 nits for untagged HDR black, but the swapchain being configured without a black point.

Fixes: https://github.com/mpv-player/mpv/issues/10558 Fixes: https://github.com/mpv-player/mpv/issues/10113