Handle old data in assumed_us_public_domain?

`KittenData` used to store the `data` field as `false` when it didn’t
have any data to store, it now assumes it will be a {}.

app/models/kitten_data.rb

@@ -45,7 +45,7 @@ def source_extras
   end
 
   def assumed_us_public_domain?
-    data.fetch(:assumptions, []).include?(:us_public_domain)
+    data.fetch(:assumptions, []).include?(:us_public_domain) if data
   end
 
   def distributions

test/unit/kitten_data_test.rb

@@ -456,6 +456,12 @@ def kitten_data
     refute kitten_data.assumed_us_public_domain?
   end
 
+  test 'assumed_us_public_domain does not crash on old kitten data instances' do
+    #kitten_data = KittenData.new(url: 'http://catalog.data.gov/some_data')
+    kitten_data.data = false # old versions stored false instead of {}
+    refute kitten_data.assumed_us_public_domain?
+  end
+
   %w[us-pd other-pd notspecified].each do |license_id|
     test "data.gov assumptions are set for federal organizations with #{license_id} license" do
       set_us_data