Prevent dropping the last label from a property graph element

Prevent dropping the last label from a property graph element

Per SQL/PGQ standard, every graph element must have at least one
label.  When dropping a label from a graph element, ensure that there
exists at least one other label on the element.  If the label being
dropped is the only label on the element, raise an error.

We hold a ShareRowExclusiveLock when modifying a property graph.
Hence the label will not be dropped even when multiple labels are
being dropped concurrently.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Reported-by: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAHg+QDeP=mTHTV48R23zKMy1SBmCKZ_L7-z5zKnYyw+K0x-gCg@mail.gmail.com