I can\'t figure out how to determine is an element that lives inside a json array is null. To check if the jsonObject itself is null, you simply use:
jsonObj
I guess json passes null values as strings, so you can't check null as a java element. Instead treat the null value as a string as check this way:
if(!mapItem.getString("date").equals("null")) { //Value is not null }
I have updated the code snippet in the original question to a working version.