fix: guard against empty data in plot_cnv_hmm_coverage_track#1307

suhr25 wants to merge 2 commits intomalariagen:mastermalariagen/malariagen-data-python:masterfrom

suhr25:fix/cnv-hmm-empty-data-crashsuhr25/malariagen-data-python:fix/cnv-hmm-empty-data-crashCopy head branch name to clipboard