If a range-partitioned table has no default partition, reject null keys.
Commit
4e5fe9ad19e14af360de7970caa8b150436c9dec introduced this
problem. Also add a test so it doesn't get broken again.
Report by Rushabh Lathia. Fix by Amit Langote. Reviewed by Rushabh
Lathia and Amul Sul. Tweaked by me.
Discussion: http://postgr.es/m/CAGPqQf0Y1iJyk4QJBdMf=pS9i6Q0JUMM_h5-qkR3OMJ-e04PyA@mail.gmail.com