a slighly uncanny `UndefRefError` with default-empty SortedSet

Hello!

while debugging one certain package (ConstraintTrees.jl) we found the following surprise with 0.19.4:

julia> union(SortedSet(Symbol[:x,:y]),SortedSet(Any[]))
SortedSet{Symbol, Base.Order.ForwardOrdering} with 2 elements:
  :x
  :y

julia> union(SortedSet(Any[]),SortedSet(Symbol[:x,:y]))    # this triggered our tests
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getproperty
   @ ./Base.jl:49 [inlined]
 [2] DataStructures.BalancedTree23{…}(::Val{…}, iterable::Base.Generator{…}, ord::Base.Order.ForwardOrdering, allowdups::Bool)
   @ DataStructures ~/.julia/packages/DataStructures/9m6yb/src/balanced_tree.jl:1053
 [3] SortedSet
   @ ~/.julia/packages/DataStructures/9m6yb/src/sorted_set.jl:82 [inlined]
 [4] packcopy
   @ ~/.julia/packages/DataStructures/9m6yb/src/sorted_set.jl:697 [inlined]
 [5] union(m1::SortedSet{Any, Base.Order.ForwardOrdering}, others::SortedSet{Symbol, Base.Order.ForwardOrdering})
   @ DataStructures ~/.julia/packages/DataStructures/9m6yb/src/sorted_set.jl:360
 [6] top-level scope
   @ REPL[18]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> union(SortedSet(),SortedSet())         # simplest reproducer
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getproperty
   @ ./Base.jl:49 [inlined]
 [2] DataStructures.BalancedTree23{…}(::Val{…}, iterable::Base.Generator{…}, ord::Base.Order.ForwardOrdering, allowdups::Bool)
   @ DataStructures ~/.julia/packages/DataStructures/9m6yb/src/balanced_tree.jl:1053
 [3] SortedSet
   @ ~/.julia/packages/DataStructures/9m6yb/src/sorted_set.jl:82 [inlined]
 [4] union(s1::SortedSet{Any, Base.Order.ForwardOrdering}, others::SortedSet{Any, Base.Order.ForwardOrdering})
   @ DataStructures ~/.julia/packages/DataStructures/9m6yb/src/sorted_set.jl:378
 [5] top-level scope
   @ REPL[19]:1
Some type information was truncated. Use `show(err)` to see complete types.

This was triggered by ConstraintTrees testing code and we can confirm that this "worked" originally in DataStructures 0.18-ish. Notably, other sets (OrderedSet) do not seem to have the issue.

I thought I'd leave a note here just in case this would point to a bigger issue somewhere (UndefRefError sounded scary). I looked at the changelogs but there's no notice of this being undefined behavior or so. I hope I didn't miss some obvious warning somewhere.

(also thanks for making DataStructures! I ❤️ the ordered ones!)

cc @stelmo the original discoverer

Start with the minimal union(SortedSet(), SortedSet()) reproduction and inspect sorted_set.jl at lines 82, 360, 378, and 697, then follow the constructor in balanced_tree.jl at line 1053. Compare the empty and non-empty union cases and the reported OrderedSet behavior. Done means empty SortedSet unions no longer raise UndefRefError and the shown non-empty unions still work.