Render group operation without non synthetic fields correctly. [DATAMONGO-759]

[spring-projects-issues]

Thomas Darimont opened DATAMONGO-759 and commented

If no fields are given as the group keys the group id is incorrectly rendered as ```
_id: {}

As a fix we should emit ```
_id: null
``` if no fields are given as the group key

---

Reference URL: http://stackoverflow.com/questions/18749596/how-to-convert-this-query-using-spring-mongodb-aggregation-apis

Backported to: 1.3.2 (Babbage SR1)

[spring-projects-issues]

Thomas Darimont commented

Added PR: #73

[spring-projects-issues]

Oliver Drotbohm commented

Feedback submitted for the pull request

[spring-projects-issues]

Thomas Darimont commented

Updated pull request