optimize: jbigs2 fix

src/ocrmypdf/_optimize.py

@@ -182,6 +182,8 @@ def convert_to_jbig2(pike, jbig2_groups, root, log, options):
             max_workers=options.jobs) as executor:
         futures = []
         for group, xrefs in jbig2_groups.items():
+            if len(xrefs) == 0:
+                continue
             prefix = 'group{:08d}'.format(group)
             cmd = ['jbig2', '-b', prefix, '-s', '-p']
             cmd.extend(make_img_name(root, xref) for xref in xrefs)