`#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, 1 insertions, 1 deletions

diff --git a/activemodel/lib/active_model/conversion.rb b/activemodel/lib/active_model/conversion.rb
index 6b0a752566..0a19ef686d 100644
--- a/activemodel/lib/active_model/conversion.rb
+++ b/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.