support/graphs: fix comparisons agains None

As Samuel said:
    In Python, None is a singleton, and it is recommended to use "is" or
    "is not" for testing them [1].

    [1] http://legacy.python.org/dev/peps/pep-0008/#programming-recommendations



Reported-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>