Replace ContactMethod::BLANK() by ContactMethod::ContactMethod(),
the default constructor.

This is the general way to construct a instance with "defaults"
behaviours, like BLANK tries to do.

Also removes the nasty usage of BLANK:
- it returns a constant instance
- the code uses it with a const_cast<>, removing the only protection!
No sense and extremly dangerous to operate with such global and
non protected instance.

Side effect: Call::peerContactMethod() returns a nullptr
and not a valid and !!mutable!! global const instance if no cm exists.

Change-Id: I8a29eacad657b76a7670055f27afeb6db98d78dc
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>