Initialize CommitterMap with empty default properties

[mmv08]

This PR:

• Fixes Cannot read property 'push' of undefined #28
• Change the interface for CommitterMap not to include optional properties
• Remove usage of non-null assertion

The problem was that if a pull request included a commit from user with no github account, it would try to push the user object to committersMap.unknown but unknown array was not preinitialised and thus undefined

[CLAassistant]

All committers have signed the CLA.

[ibakshay]

@mikheevm, That is a great catch 💯 . Thank you very much :).