Some tests for no-leaf-samples

[jeromekelleher]

Digging out some obscure issues when we have no leaf samples in a pedigree.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.23%. Comparing base (0c86a4f) to head (20d666d).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/msprime.c	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2388   +/-   ##
=======================================
  Coverage   91.23%   91.23%           
=======================================
  Files          20       20           
  Lines       11816    11828   +12     
  Branches     2293     2295    +2     
=======================================
+ Hits        10780    10791   +11     
  Misses        568      568           
- Partials      468      469    +1     

Flag	Coverage Δ
C	91.23% <91.66%> (+<0.01%)	⬆️
python	98.68% <ø> (ø)

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