[Bug][patch]: After dropping the last label from a property graph element, invoking pg_get_propgraphdef() triggers an assertion failure

Hi hackers,

ALTER PROPERTY GRAPH ... ALTER ... DROP LABEL currently allows removing
the last label from an element, leaving it with zero labels.

On assert-enabled builds, pg_get_propgraphdef() hits
TRAP: failed Assert("count > 0"), File: "ruleutils.c", Line: 1837, PID:
1821840

Repro:

CREATE TABLE t (x int PRIMARY KEY, y int, z int);
CREATE PROPERTY GRAPH g VERTEX TABLES (t KEY (x) LABEL l1 LABEL l2);
ALTER PROPERTY GRAPH g ALTER VERTEX TABLE t DROP LABEL l2;
ALTER PROPERTY GRAPH g ALTER VERTEX TABLE t DROP LABEL l1;
SELECT pg_get_propgraphdef('g'::regclass);

We can fix it two ways, (1) Prevent dropping the last label; (2) handle
zero labels.
I feel it is easier to prevent dropping the last label than handling zero
labels. Thoughts?

The attached patch adds a check in AlterPropGraph() before
performDeletion(). It scans pg_propgraph_element_label to count labels
for the element, and raises an error if only one remains. A regression test
is included
that drops labels down to the last one, verifies the error, then re-adds
them back.

Thanks,
Satya