Add nuvigator maybeOf

[raapperez]

Splits the nullOk logic in two:

• Nuvigator.of => Should be used when its certain that the Nuvigator is available
• Nuvigator.maybeOf => The existence of the Nuvigator is not required, and null return type is accepted.

Change from Map<String, Object>? initialArguments to Map<String, dynamic>? initialArguments as internally there is a cast to Map<String, dynamic>, it makes more sense.