Make fields of type Array or Hash initialize to empty Array or Hash (instead of nil)

I found this to be very useful, but I don't know if it's in line with your overall strategy.

**Explanation:** When using relations, a "many" reference (e.g.: has_many, embeds_many) initializes as an empty array, which allows you to use array-specific methods (e.g.: `Array#<<`) with no worries.  This behavior is nice.  When defining a regular field of type Array, it initializes as nil, so if you're not sure whether the array already has elements in it, then you can't use array-specific methods without first testing whether it actually *is* an array.  A similar argument applies to operating on fields of type Hash.  This update just sets Array and Hash fields as "empty" instead of nil.