Fix PhyDatToMatrix crash on zero-character phyDat

[ms609]

PhyDatToMatrix() crashed when given a phyDat with nr=0 (e.g. from a star tree) because matrix() could not reconcile empty data with non-empty dimnames. Add an early return for the nr=0 case, returning a 0-column matrix with correct row names.

This also fixes AddUnconstrained() when passed such a phyDat, since it delegates to PhyDatToMatrix() internally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.36%. Comparing base (5bb40b8) to head (fe21dfc).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #261   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
=======================================
  Files          78       78           
  Lines        5774     5777    +3     
=======================================
+ Hits         5564     5567    +3     
  Misses        210      210           

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