This is useful for VCL simplification

Ref: https://github.com/varnishcache/varnish-cache/pull/2680

ref: 0c74d1b99f1f43fc99725fe14322760afa7858ee

When cluster_task_param_l as called with black==0, we would
fail to alloc enough space for the existing blacklist, so
the blacklist would get overwritten by the next allocation
on the workspace

We accidentally used the presence of the argument, not the argument
value itself

or elsewhere, required for resolve=NOW where we do not have
a parameter object available.