Implement is_empty method

[hyanwong]

Description

Fixes #2640

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[codecov]

Codecov Report

Merging #2668 (ece21ff) into main (ec992da) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2668   +/-   ##
=======================================
  Coverage   93.91%   93.91%           
=======================================
  Files          27       28    +1     
  Lines       27657    27692   +35     
  Branches     1270     1274    +4     
=======================================
+ Hits        25973    26008   +35     
  Misses       1647     1647           
  Partials       37       37           

Flag	Coverage Δ
c-tests	92.25% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	72.23% <11.11%> (+0.01%)	⬆️
python-tests	98.97% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
python/tskit/trees.py	98.75% <100.00%> (+<0.01%)	⬆️
python/tskit/__init__.py	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec992da...ece21ff. Read the comment docs.

[benjeffery]

Thanks @hyanwong! LGTM!

[jeromekelleher]

Looks good, but I don't really see the value in the more complicated definition. This should have another name (if we need it at all).

[hyanwong]

We couldn't decide on a sensible name ("branchless" was best), but came to the conclusion that tree.num_edges == 0 is pretty clear, so killing this idea unless there's greater demand. Best would be to document this somewhere though