Handle missing metadata

[ErinWeisbart]

Should fix https://github.com/broadinstitute/CP186-A549-WG/issues/4

Recipe errored at 1./0.merge-single-cells because a site had foci but no cells. So metadata_{site}.csv made during 0./2.process-cells was empty but a non-empty csv was expected in 1./0.merge-single-cells because there was site information in the input_spotdir.

This PR changes:

• 0./2.process-cells to not save empty csvs
• 1./0.merge-single-cells to skip a site with empty or missing metadata_{site}.csv
• sanitize_gene_col() to avoid errors trying to process an empty metadata_df

[ErinWeisbart]

And apparently also includes a commit that should fix an error in 0./4.image-and-segmentation-qc. Don't know how I put it on this branch, but I'm too scared of messing things up to move it so here it stays....