diff estimator: don't crash at empty feature tree

[craigds]

If user deletes all the features in a dataset, we end up with a tree that looks like

datasetname/feature/A/A/A/A

... with no features in it. The final A is an empty tree.

This causes the following crash in diff:

$ kart diff --only-feature-count=medium
4b825dc642cb6eb9a060e54bf8d69288fbee4904...HEAD -o json
kart helper: unhandled exception
Traceback (most recent call last):
  File "kart/helper.py", line 240, in helper
  File "click/core.py", line 1157, in __call__
  File "click/core.py", line 1078, in main
  File "kart/cli_util.py", line 72, in invoke
  File "click/core.py", line 1688, in invoke
  File "click/core.py", line 1434, in invoke
  File "click/core.py", line 783, in invoke
  File "click/decorators.py", line 34, in new_func
  File "kart/diff.py", line 194, in diff
  File "kart/diff.py", line 34, in feature_count_diff
  File "kart/diff_estimation.py", line 156, in
estimate_diff_feature_counts
  File "kart/diff_estimation.py", line 60, in
get_approximate_diff_blob_count
  File "kart/tabular/v3_paths.py", line 374, in diff_estimate
  File "kart/tabular/v3_paths.py", line 331, in
_recursive_depth_first_diff_estimate
  File "kart/tabular/v3_paths.py", line 331, in
_recursive_depth_first_diff_estimate
  File "kart/tabular/v3_paths.py", line 331, in
_recursive_depth_first_diff_estimate
  [Previous line repeated 1 more time]
  File "kart/tabular/v3_paths.py", line 321, in
_recursive_depth_first_diff_estimate
IndexError: list index out of range

This change fixes the issue so that command can deal with these repos.

Whether they should have empty trees committed to them is another matter I guess

Checklist:

• [x] Have you reviewed your own change?
• [x] Have you included test(s)?
• [ ] Have you updated the changelog?