Property graph: fix error handling when dropping non-existent label property

Hi,

I am testing graph tables today and noticed an improper error message when dropping a property.

Here is a simple repro:
```
evantest=# CREATE TABLE t1 (a int PRIMARY KEY, b int);
CREATE TABLE
evantest=#
evantest=# CREATE PROPERTY GRAPH g
evantest-# VERTEX TABLES (
evantest(# t1
evantest(# LABEL l1 PROPERTIES (a AS p1)
evantest(# LABEL l2 PROPERTIES (b AS p2)
evantest(# );
CREATE PROPERTY GRAPH
evantest=#
evantest=# ALTER PROPERTY GRAPH g
evantest-# ALTER VERTEX TABLE t1
evantest-# ALTER LABEL l1 DROP PROPERTIES (p2);
ERROR: could not find tuple for label property 0
```

This does not look like a normal user-facing SQL error message.

Looking into the code, AlterPropGraph() checks whether the property exists in the graph, but does not check if label property exists. As a result, InvalidOid can be passed to ObjectAddressSet() and then to performDeletion().

The fix is simple, just check the label-property OID before trying to delete it. I also added a regress test.

With the patch, the error becomes:
```
evantest=# ALTER PROPERTY GRAPH g
evantest-# ALTER VERTEX TABLE t1
evantest-# ALTER LABEL l1 DROP PROPERTIES (p2);
ERROR: property graph "g" element "t1" label "l1" has no property "p2"
```

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/