Use <unnamed> for name of unnamed portal's memory context

Otherwise just printing an empty string makes the memory context debug
output slightly confusing.

Discussion: https://www.postgresql.org/message-id/flat/ccb353ef-89ff-09b3-8046-1d2514624b9c%402ndquadrant.com

src/backend/utils/mmgr/portalmem.c

@@ -220,8 +220,8 @@ CreatePortal(const char *name, bool allowDup, bool dupSilent)
 	/* put portal in table (sets portal->name) */
 	PortalHashTableInsert(portal, name);
 
-	/* reuse portal->name copy */
-	MemoryContextSetIdentifier(portal->portalContext, portal->name);
+	/* for named portals reuse portal->name copy */
+	MemoryContextSetIdentifier(portal->portalContext, portal->name[0] ? portal->name : "<unnamed>");
 
 	return portal;
 }