Don't warn about missing DWIs if --dwi_dir is given

[tkkuehn]

The problem

If --dwi_dir is passed on the command line, we don't expect to find any DWIs in the bids_dir. We still warn though, so it would be nice to suppress that warning somehow. https://github.com/akhanf/snakebids/issues/293 will help with this.

[kaitj]

Just to add a note (and may be better for a separate issue), but we should update to be able to handle different naming schemes (re: see changes for to handle snakedwi).