inconsistent behavior of lower_bound and upper_bound

[jbangert]

[19] pry(#Elf::Writer::Layout)> x= RBTree.new
=> #<RBTree: {}, default=nil, cmp_proc=nil>
[20] pry(#Elf::Writer::Layout)> x.lower_bound(1)
=> []
[21] pry(#Elf::Writer::Layout)> x[2]="Hello"
=> "Hello"
[22] pry(#Elf::Writer::Layout)> x.lower_bound(1)
=> [2, "Hello"]
[23] pry(#Elf::Writer::Layout)> x.upper_bound(1)
=> nil
[24] pry(#Elf::Writer::Layout)> x.lower_bound(3)
=> nil

lower_bound and upper_bound returns an empty array on an empty tree, but a nil otherwise.