Add support for undef initializer in ArrayPartition

[JoshuaLampert]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Makes it possible to initialize an ArrayPartition with undefs if it only consists of one partition. This came up in NumericalMathematics/DispersiveShallowWater.jl#207 (comment). A more minimal example extracted from that PR is:

using LinearAlgebra, SparseArrays
using OrdinaryDiffEqSDIRK
using SciMLOperators
using RecursiveArrayTools

function rhs_explicit!(du, u, p, t)
    du .= u
end

n = 2
op = MatrixOperator(1.0 * sparse(I, n, n))
func = SplitFunction(op, rhs_explicit!)
u0 = ones(n)
u0 = ArrayPartition(u0)
prob = SplitODEProblem{true}(func, u0, (0.0, 1.0))
sol = solve(prob, KenCarp4(), saveat = 0.1)

Note that this MWE still fails at a later stage, but I guess this is something that would need to be fixed in LinearSolve.jl.