Merge branch 'cherry-pick-dfc75f37-21.1' into '21.1'

Sweeping !24861 from master to 21.1.
Explicitly check the validity of the objects before asking for their elements in diff_root_files.py

See merge request !24929

Tools/PyUtils/python/scripts/diff_root_files.py

@@ -261,7 +261,7 @@ def main(args):
             read_old = True
             read_new = True
 
-            if (args.order_trees and d_old[-1] == d_new[-1]) or d_old == d_new:
+            if (args.order_trees and d_old and d_new and d_old[-1] == d_new[-1]) or d_old == d_new:
                 n_good += 1
                 continue