python: fix Message.get_header

8dc8495010057202b725ac029831c03f4e3ab6bd introduced a bug, if the
requested header is not set the underlying notmuch function returns an
empty string that also made the expression true resulting in an
exception being raised. Partly revert the commit to fix this
issue. Testing for equality with None is correct in this case since
the restype of the function Message._get_header is c_char_p so NULL
pointers are in fact converted to None in this case.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>