Do not trip over empty hashes in responses

When objects contained an empty hash, for example a broker of type 'puppet'
with no additional configuration, Razor::CLI::Format::format_Default_object
would attempt to calculate [].max which fails miserably.

Now, we shortcircuit that and print empty hashes as '{}'

Similar to https://github.com/puppetlabs/razor-client/pull/9 submitted by
https://github.com/derekmwright