impl-port: don't send NULL tag to clear

NULL tags are never handled and so tags just keeps configured on the
ports. It's also hard to know the direction from the NULL data.

Instead, send an empty Tag with the direction to clear everything.

src/pipewire/impl-port.c

@@ -1954,7 +1954,7 @@ int pw_impl_port_recalc_tag(struct pw_impl_port *port)
 			count++;
 		}
 	}
-	param = count == 0 ? NULL : spa_tag_build_end(&b.b, &f);
+	param = spa_tag_build_end(&b.b, &f);
 
 	old = port->tag[direction];