Write a spec that triggers the bug I'm trying to fix

The expectation is that the code won't trigger an exception, but it
does:

    ActiveJSON::Base.create does not fail when the loaded JSON was empty
    Failure/Error: Empty.create()
    NoMethodError:
      undefined method `length' for nil:NilClass
    # ./lib/active_hash/base.rb:135:in `insert'
    # ./lib/active_hash/base.rb:497:in `save'
    # ./lib/active_hash/base.rb:174:in `create'
    # ./spec/active_json/base_spec.rb:51:in `block (3 levels) in <top (required)>'

spec/active_json/base_spec.rb

@@ -46,6 +46,12 @@ class Empty < ActiveJSON::Base ; end # Empty JSON
     end
   end
 
+  describe ".create" do
+    it "does not fail when the loaded JSON was empty" do
+      Empty.create()
+    end
+  end
+
   describe ".delete_all" do
     context "when called before .all" do
       it "causes all to not load data" do