Fix traceback in handling of empty container.yaml and content_sets.yml.
    
    When file is empty, yaml.safe_load returns `None` which Freshmaker did
    not expect.
    
    This also puts the `ContainerImage.resolve()` code into try/except block,
    so similar issues in this part of code won't mark the whole Freshmaker
    Event as failed, but only marks particular ContainerImage as failed.