models: Fix warning about memebership check (#503)
models: Fix warning about memebership check (#503)

Previously the value 'fields' of a Model could be potentially `None`
which is not usuable for `x in y` kind of checks. This raised a pylint
warning.
This patch fixes this minority.

Signed-off-by: Vinzenz Feenstra <600e8a10eeff4f08c65e2cf493cd851025e86fd2@redhat.com>
leapp-to/prototype,leapp-to/prototype,leapp-to/prototype,leapp-to/prototype