What is the convention in JSON for empty vs. null?
"JSON has a special value called null which can be set on any type of data including arrays, objects, number and boolean types."
"The JSON empty concept applies for arrays and objects...Data object does not have a concept of empty lists. Hence, no action is taken on the data object for those properties."
Here is my source.
It is good programming practice to return an empty array [] if the expected return type is an array. This makes sure that the receiver of the json can treat the value as an array immediately without having to first check for null. It's the same way with empty objects using open-closed braces {}.
Strings, Booleans and integers do not have an 'empty' form, so there it is okay to use null values.
This is also addressed in Joshua Blochs excellent book "Effective Java". There he describes some very good generic programming practices (often applicable to other programming langages as well). Returning empty collections instead of nulls is one of them.
Here's a link to that part of his book:
http://jtechies.blogspot.nl/2012/07/item-43-return-empty-arrays-or.html
There is the question whether we want to differentiate between cases:
"phone" : "" = the value is empty
"phone" : null = the value for "phone" was not set yet
If we want differentiate I would use null for this. Otherwise we would need to add a new field like "isAssigned" or so. This is an old Database issue.
Empty array for empty collections and null for everything else.