Revert part of r280687, reporting "1" (true) for empty value.

For example, it made gpart partitions without label report "1" as label.

PR:		202089
MFC after:	3 days

lib/libgeom/geom_xml2tree.c

@@ -286,7 +286,7 @@ EndElement(void *userData, const char *name)
 			    "element", name);
 			return;
 		}
-		gc->lg_val = p ? p : strdup("1");
+		gc->lg_val = p;
 		LIST_INSERT_HEAD(c, gc, lg_config);
 		return;
 	}