Allow passing None for username in v2.Password

None must be an acceptable parameter for username in password due to
tests in other libraries, however we should still raise an error if
neither username or user_id is passed. Use and check a sentinel value
instead of None.

Change-Id: Id61cfd1423afa8f9dd964fda278f4fab40887512
Closes-Bug: #1361444
Merge "Allow passing None for username in v2.Password"