Make it recognize not null constraints and set type to Unknown when the value is not recognized.

Note column not null constraints where not stored in pg_constraint before postgresl 18

pglablib/catalog/PgConstraint.h

@@ -11,12 +11,14 @@
 #include <vector>
 
 enum class ConstraintType {
+    Unknown, // not recognized, new???
 	PrimaryKey, // p
 	ForeignKey, // f
 	Unique, // u
 	Check,  // c
 	ConstraintTrigger, // t
 	ExclusionConstraint, // x
+    NotNull, // n
 };
 
 void operator<<(ConstraintType &s, const Pgsql::Value &v);