HashTraits<RefPtr> uses 'nullptr'

This is a follow up to the recently landed
https://codereview.chromium.org/170603003.
The 'HashTraits<RefPtr>' class now uses 'nullptr'
to represent an empty value.
This brings following improvements:
- consistency with 'HashTraits<OwnPtr>'
- the trait code is more understandable
  (passOut(std::nullptr_t); // Apparenlty handles an empty value.
   vs
   passOut(P* value); // Hm.. Why is that neeeded? ).

Review URL: https://codereview.chromium.org/177423003

git-svn-id: svn://svn.chromium.org/blink/trunk@167770 bbb929c8-8fbe-4397-9dbb-9b2b20218538