Clarify tree.mrca if MRCA does not exist

[hyanwong]

No description provided.

[codecov]

Codecov Report

Merging #2800 (710bba4) into main (f7ba548) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2800   +/-   ##
=======================================
  Coverage   89.93%   89.93%           
=======================================
  Files          30       30           
  Lines       29171    29171           
  Branches     5697     5697           
=======================================
  Hits        26236    26236           
  Misses       1666     1666           
  Partials     1269     1269           

Flag	Coverage Δ
c-tests	86.42% <ø> (ø)
lwt-tests	80.13% <ø> (ø)
python-c-tests	67.12% <ø> (ø)
python-tests	99.02% <ø> (ø)

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Impacted Files	Coverage Δ
python/tskit/trees.py	98.66% <ø> (ø)

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