`MomentOverride.defaultFormat` null if not defined in config

[jfdnc]

Fixes https://github.com/adopted-ember-addons/ember-moment/issues/362

Current implementation throws TypeError for undefined if config.moment.outputFormat is not specified.

• checks for presence of config.moment and config.moment.outputFormat
  • set value if present, fall back to null otherwise

[jfdnc]

@alexlafroscia Thanks for the review and the lint fix 🎉