only generate RID during `maintain_row()` if it is currently NULL
only generate RID during `maintain_row()` if it is currently NULL

This change is in preparation to play around with new service API options
to allow privileged clients to assign RIDs for some rows, e.g. when
importing table dumps from a prior catalog.  Details TBD.

informatics-isi-edu/ermrest,informatics-isi-edu/ermrest