pipewire: allow NULL pointers in pw_properties_free()

Just like the real free() we should just ignore a NULL pointer, makes the
caller code easier for those instances where properties are optional.

Patch generated with concinelle with a few manual fixes.

src/modules/module-adapter/adapter.c

@@ -300,8 +300,7 @@ struct pw_impl_node *pw_adapter_new(struct pw_context *context,
 	return node;
 
 error:
-	if (props)
-		pw_properties_free(props);
+	pw_properties_free(props);
 	errno = -res;
 	return NULL;
 }