Please let's have null-safe version of Twig extension #76

Since 3.0, parameter was typehinted to string, which breaks stuff for everybody passing null there. Nullable getters are very widespread, please don't make us do
{{ activity.getNote ? activity.getNote|purify }} on each instance. It's fine this typehint is present in HTMLPurifierRuntime, but twig filter should be a closure typehinted to ?string that shouldn't pass null along to HTMLPurifierRuntime