#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>'
```

activemodel/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   `#to_param` returns `nil` if `#to_key` returns `nil`. Fixes #11399.
+
+    *Yves Senn*
+
 *   Ability to specify multiple contexts when defining a validation.
 
     Example:

activemodel/lib/active_model/conversion.rb

@@ -62,7 +62,7 @@ module ActiveModel
     #   person = Person.create
     #   person.to_param # => "1"
     def to_param
-      persisted? ? to_key.join('-') : nil
+      (persisted? && key = to_key) ? key.join('-') : nil
     end
 
     # Returns a +string+ identifying the path associated with the object.

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,