Update the instance method `last()` on array objects to not panic when used with an empty array

[lara]

• Solves https://github.com/goby-lang/goby/issues/853:

  a = []
  a.last # => nil

• Adds more tests to ensure the correct types are returned based on whether an argument is passed in or not to the last() method:

a = [2, 6, 3, 4]
a.last    # => 4
a.last(1) # => [4]

[codecov[bot]]

Codecov Report

Merging #854 into master will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #854      +/-   ##
==========================================
+ Coverage   81.02%   81.06%   +0.03%     
==========================================
  Files          54       54              
  Lines        7496     7498       +2     
==========================================
+ Hits         6074     6078       +4     
+ Misses       1188     1187       -1     
+ Partials      234      233       -1     

Impacted Files	Coverage Δ
vm/array.go	96.65% <100.00%> (+0.33%)	:arrow_up:

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