[REQ][JAVA]: Assign collections to null by default in auto-generated POJO

Created by: sonalidhome

Description:

Currently /JavaJaxRS/spec/pojo.mustache instantiate the collections. It would be great if those get assigned to null by default and instantiate if required.
There is a PR: #3615 which works similarly.

Proposed Solution:

Updating /JavaJaxRS/spec/pojo.mustache template referring to the above-mentioned PR.