don't sort! parents when empty in order()

[mattwigway]

This avoids the following error which crashes the scheduler:

MethodError: defalg(::Vector{Union{}}) is ambiguous. Candidates:
  defalg(v::AbstractArray{<:Union{Missing, Number}}) in Base.Sort at sort.jl:658
  defalg(::AbstractArray{<:Union{Missing, String1, String15, String3, String7}}) in InlineStrings at /Users/juliauser/.julia/packages/InlineStrings/C5tOh/src/InlineStrings.jl:656
Possible fix, define
  defalg(::AbstractArray{<:Missing})

When parents is empty in order(), it has type Vector{Union{}}. Since Union{} is a subtype of all
Union{...any types here...} types, any package that defines Base.defalg(AbstractArray{<:Union{...any type...}})
will cause an ambiguity error. This is true even without type piracy, as the Union in the other package could
consist entirely of types defined in that package.

@jpsamaroo

[jpsamaroo]

Thanks for patching this!

[jpsamaroo]

Thanks!