[Feature #19830] Allow Array#transpose to take an optional element size argument

[tomstuart]

One benefit of supplying an initial value to Enumerable#inject is that it avoids an annoying edge case when the collection is empty:

>> [1, 2, 3].inject(:+)
=> 6 # good

>> [].inject(:+)
=> nil # bad

>> [].inject(0, :+)
=> 0 # good

A similar edge case exists for Array#transpose:

>> [[1, :a], [2, :b], [3, :c]].transpose
=> [[1, 2, 3], [:a, :b, :c]] # good

>> [].transpose
=> [] # bad

Although no explicit nil is produced here, the subtle problem is that the caller may assume that the result array contains arrays, and that assumption leads to nils in the empty case:

>> [[1, :a], [2, :b], [3, :c]].transpose.then { _2.join }
=> "abc"

>> [].transpose.then { _2.join }
undefined method `join' for nil:NilClass (NoMethodError)

If we allow Array#transpose to take an optional argument specifying the size of the result array, we can use this to always return an array of the correct size:

>> [[1, :a], [2, :b], [3, :c]].transpose(2)
=> [[1, 2, 3], [:a, :b, :c]] # good

>> [].transpose(2)
=> [[], []] # good

By avoiding an unexpectedly empty result array, we also avoid unexpected downstream nils:

>> [[1, :a], [2, :b], [3, :c]].transpose(2).then { _2.join }
=> "abc"

>> [].transpose(2).then { _2.join }
=> ""

This PR adds an optional argument to Array#transpose to support the above usage.

Something similar was requested eleven years ago in feature #6852. I believe this PR addresses the problem expressed in that issue without compromising backward compatibility with existing callers of #transpose.