handle "null" values in config.yaml

When config.yaml has "REPO_MIRROR_SERVER_HOSTNAME: null" in it, which it will in most cases, the config-tool prints an error but does not proceed further with any checks (no summary table is printed).

For cases where a config.yaml contains unnecessary entries, such as above, perhaps the config-tool could print an informational message and if that config is saved through the config-tool that value would not be saved.