Add dedicated printing for empty combinatorial objects

[lgoettgens]

Resolves #4229.

[joschmitt]

I don't expect that I will see these prints very often in my day-to-day life, so anything is fine for me :)

Anyway, this works (and seems what @JohnAAbbott wished for?), so approval from me.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.58%. Comparing base (7b49bb4) to head (9327676).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
...inatorics/EnumerativeCombinatorics/compositions.jl	60.00%	2 Missing ⚠️
...mbinatorics/EnumerativeCombinatorics/partitions.jl	60.00%	2 Missing ⚠️
...rics/EnumerativeCombinatorics/weak_compositions.jl	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4236      +/-   ##
==========================================
- Coverage   84.58%   84.58%   -0.01%     
==========================================
  Files         631      631              
  Lines       85012    85024      +12     
==========================================
+ Hits        71911    71919       +8     
- Misses      13101    13105       +4     

Files with missing lines	Coverage Δ
...inatorics/EnumerativeCombinatorics/compositions.jl	93.98% <60.00%> (-1.37%)	⬇️
...mbinatorics/EnumerativeCombinatorics/partitions.jl	96.42% <60.00%> (-0.52%)	⬇️
...rics/EnumerativeCombinatorics/weak_compositions.jl	90.76% <60.00%> (-2.68%)	⬇️

... and 1 file with indirect coverage changes