Provide empty mock for dgram module

Created by: micopiira

dgram is an node module that provides an implementation of UDP Datagram sockets.

I'm using a package that depends on dgram, but does not need it for the most part. Providing an empty mock for dgram; just like there are empty mocks for fs, net and tls modules would save me from ejecting like in #672.

This should be as simple as adding line: dgram: 'empty' (NOTE: I have not tested this yet!) to the webpack configuration files, I can also provide a PR if you want.