DASH and HLS fail trying to merge segments if `--skip-dl` is used

[rlaphoenix]

Describe the bug Since no segments will be downloaded, the HLS and DASH downloaders get confused and have no segments to merge.

Expected behavior This is easily fixable on HLS, but for DASH a slight bit more effort needs to be put in to keep --export supported.

Screenshots N/A

Additional context N/A