Conditionally enable / disable no-null based on presence of graphql

[manovotny]

The unicorn/no-null rule was added in v6, and while it is the idyllic state, it does appear to complete / conflict with GraphQL standards / best practices. To the point that even if you pass undefined to GraphQL, it’ll coerce it to null anyway.

To that end, we should check for the presence of graphql in dependencies and conditionally disable this rule.

Might be worth waiting until #37 too.

[manovotny]

This will be fixed in the v7 release.