Bugfix: let integer_partitions(0) == [Int[]]

[inkydragon]

Fix #138, and half of #143

GAP result:
NOTE: we use the same order as GPA do.

GAP 4.11.1 of 2021-03-02

gap> Partitions( 0 );
[ [  ] ]
gap> Partitions( 1 );
[ [ 1 ] ]
gap> Partitions( 2 );
[ [ 1, 1 ], [ 2 ] ]
gap> Partitions( 5 );
[ [ 1, 1, 1, 1, 1 ], [ 2, 1, 1, 1 ], [ 2, 2, 1 ], [ 3, 1, 1 ], [ 3, 2 ], [ 4, 1 ], [ 5 ] ]

Wolfram:

In[2]:= IntegerPartitions[0]
Out[2]= {{}}
In[4]:= IntegerPartitions[1]
Out[4]= {{1}}
In[5]:= IntegerPartitions[2]
Out[5]= {{2},{1,1}}
In[6]:= IntegerPartitions[5]
Out[6]= {{5},{4,1},{3,2},{3,1,1},{2,2,1},{2,1,1,1},{1,1,1,1,1}}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.22%. Comparing base (bbd5b1b) to head (411b412).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #179   +/-   ##
=======================================
  Coverage   97.21%   97.22%           
=======================================
  Files           8        8           
  Lines         826      828    +2     
=======================================
+ Hits          803      805    +2     
  Misses         23       23           

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