Fix mistake in ceilometer parse notifications

In case of empty list of notifications it was failing
And it shouldn't fail at all.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1

iQEcBAABAgAGBQJTvIL5AAoJEDvpPTuJuon0YRwH/jahK7jXEjtZuFM/8F6F3Qtj
pQAJHQ1mfMfAAzZ2nHyKZYipOUkhd2u5BvVUz9J2ZXUJOZV1a+4iWDwvM+EmZrTX
JPWDevPuwDq8uf/D9XNhNEGYcL+mRzeRVaXeic8+bAXWNQcXgYIMiniwhx2K604+
h8t+HXTYZtZr2EqFY+GGd7f7mMQtaUCIb/DwI7qUQRYTQBVRaCClcZOYemlfZyFL
z8Aqf4O48KTHWqbIvVPyr2GG8ypRaBA2Tx9gmMqKilMM9tTptGoayC/j4zJLXIB8
eg0upb6UfsZgv6x1yPKPZRI1j8tIC7owWNRS9+etlyiMJ74/cfS4jqi5az+ouNQ=
=6yEz
-----END PGP SIGNATURE-----