What is the behavior of null/empty value in compound shard key? I have a shard key based on {Field1:1,Field2:1} where Field2 can
  be null. So if two documents have same value for Field1 and
  null/empty value of Field2 then are they considered to belong to
  the same shard key/non split-able??
In MongoDB 'null' is just another value so two documents that have the same values for field1 (some non-null) and same value for field2 (null) have the same value for the shard key and will always be in the same "chunk".