builder: fix calls to builder_add that are not terminated by NULL
Created by: tretter
spa_pod_builder_add() is a va_args function that is terminated by a NULL argument. The last argument must be a pointer type, because otherwise checking for a NULL pointer can fail.
The attribute((sentinel)) prints a compiler warning, if the last argument of a call to spa_pod_builder_add() is not a pointer type.
Fix all sentinel warnings by replacing all integer type 0 with pointer type NULL in calls to spa_pod_builder_add().