Warn about empty config

[JDNSTORM]

My gradle sync failed and the ide won't provide autocompletion for buildkonfig or any of its scope. I had to manually type and copy the imports here to make the sync successful.
Please make it so that it just warns about empty defaultConfigs or any configs. buildkonfig can be easily configured after a successful sync.

[yshrsmz]

That sounds good. would you like to send a PR?