Fix zero-dimensional cone in cones in PolyhedralGeometry

[paemurru]

By Definition 1.2.1 in [CLS11](@cite), the set {0} is a cone.
The function cones(X, 0) now returns an iterator that returns the zero-dimensional cone, provided the lineality space has length zero, fixing a bug.
If n is negative, then cones(X, n) now returns an empty iterator, fixing the type instability.

[benlorenz]

Thanks, indeed returning nothing seems wrong in that case. I have added a few suggestions for improvements.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.50%. Comparing base (aa41156) to head (53ad859).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4336   +/-   ##
=======================================
  Coverage   84.50%   84.50%           
=======================================
  Files         644      645    +1     
  Lines       85606    85624   +18     
=======================================
+ Hits        72339    72357   +18     
  Misses      13267    13267           

Files with missing lines	Coverage Δ
src/PolyhedralGeometry/PolyhedralFan/properties.jl	98.41% <100.00%> (+0.07%)	⬆️

... and 2 files with indirect coverage changes

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