don't die when trying to convert None to float

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1091360
Change-Id: I1c0e5906a2708d546dcb5c8e1d68e07294edaa69
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>