Check if the docker config is nil, fixes #13371

[tomjn]

Sometimes the host network has a nil config, causing a crash on vagrant up when checking the docker networks.

See #13371 for more details of the bug this fixes

[tomjn]

It looks like network_containing_address in the same file has a similar check that wasn't applied here:

        networks.each do |net|
          next if !net["IPAM"]
          config = net["IPAM"]["Config"]
          next if !config || config.size < 1
          config.each do |opts|

[chrisroberts]

Thanks!