Bugfix: include empty combination

[Stivanification]

Closes Issues #86 and #123. Currently the output of combinations(a) does not include the empty combination, which is a bug. Also in the tests, we should have combinations([]) = [[]], i.e., an array that contains the empty set, not the empty set itself.

[codecov[bot]]

Codecov Report

Base: 96.85% // Head: 96.85% // No change to project coverage :thumbsup:

Coverage data is based on head (7ceb259) compared to base (d1b633b). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #129   +/-   ##
=======================================
  Coverage   96.85%   96.85%           
=======================================
  Files           7        7           
  Lines         700      700           
=======================================
  Hits          678      678           
  Misses         22       22           

Impacted Files	Coverage Δ
src/combinations.jl	85.27% <100.00%> (ø)

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