Filter SQL only if another filter is not set

I'd like to apply a filter to an SQL query if and only if another filter is not set. This would look something like:

WHERE true
    [[AND status = CASE WHEN COALESCE({{special_cases}}, 0) = 0 THEN {{status}} ELSE status END]]

i.e. apply the {{status}} filter on the status column if and only if the {{special_cases}} filter is not set.

In this form the filter is ignored entirely. BTW how is the optionality [[...]] evaluated with more than one variable present?

Any ideas are welcome,