message: fix: return empty dict when no differences are found
diff: @@ -80,7 +80,7 @@ def compare_tsv_column(new, old, column_name):
  def compare_aa_muts_one(new, old, column_name):
  diff = tsv_diff(new, old, column_name)
  if diff.empty:
  - return
  + return {}
  result = {}
  for row in diff.iterrows():
  