Update default value function and remove local variable.

Implements changes as per the discussion in github.

* Remove local variable and dereference $this instead.
* Use defaultNull() instead of defaultValue(null)

* https://github.com/hwi/HWIOAuthBundle/pull/157#discussion_r3440869
Merge branch 'target_path_parameter' of https://github.com/elyobo/HWIOAuthBundle into target_path_parameter