server: test using config.get() in case dict value is None

server: test using config.get() in case dict value is None

If a key 'foo' is set to None, `if config.get('foo'):` will be false while
`if 'foo' in config:` will be true.  A None value is not useful here, so
config.get() is the better check.

Thanks to Adam Pritchard for the suggestion.