feat: subgroup enumeration with no subgroups

[thofma]

No description provided.

[thofma]

P.S.: Before

julia> G = abelian_group([2]);

julia> length(collect(subgroups(G; quotype = [3])))
1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.94%. Comparing base (ab9b6d2) to head (a56518c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1670      +/-   ##
==========================================
- Coverage   75.97%   75.94%   -0.03%     
==========================================
  Files         361      361              
  Lines      113532   113783     +251     
==========================================
+ Hits        86259    86418     +159     
- Misses      27273    27365      +92     

Files with missing lines	Coverage Δ
src/GrpAb/SubgroupEnum.jl	84.02% <100.00%> (+0.14%)	⬆️

... and 58 files with indirect coverage changes