`#to_param` returns `nil` if `to_key` returns `nil`. Closes #11399.

The documentation of `#to_key` (http://api.rubyonrails.org/classes/ActiveModel/Conversion.html#method-i-to_key)
states that it returns `nil` if there are no key attributes. `to_param` needs
to be aware of that fact and return `nil` as well.

Previously it raised the following exception:

```
  1) Error:
ConversionTest#test_to_param_returns_nil_if_to_key_is_nil:
NoMethodError: undefined method `join' for nil:NilClass
    /Users/senny/Projects/rails/activemodel/lib/active_model/conversion.rb:65:in `to_param'
    /Users/senny/Projects/rails/activemodel/test/cases/conversion_test.rb:34:in `block in <class:ConversionTest>'
```

1 files changed, 10 insertions, 0 deletions

diff --git a/activemodel/test/cases/conversion_test.rb b/activemodel/test/cases/conversion_test.rb
index 3bb177591d..c5cfbf909d 100644
--- a/activemodel/test/cases/conversion_test.rb
+++ b/activemodel/test/cases/conversion_test.rb
@@ -24,6 +24,16 @@ class ConversionTest < ActiveModel::TestCase
     assert_equal "1", Contact.new(id: 1).to_param
   end
 
+  test "to_param returns nil if to_key is nil" do
+    klass = Class.new(Contact) do
+      def persisted?
+        true
+      end
+    end
+
+    assert_nil klass.new.to_param
+  end
+
   test "to_partial_path default implementation returns a string giving a relative path" do
     assert_equal "contacts/contact", Contact.new.to_partial_path
     assert_equal "helicopters/helicopter", Helicopter.new.to_partial_path,