Mark params autodetection in generic way.

For this make Ast detect no params as nil instead of [].
Similarly detect no explicit params as nil not [].
This allows the auto-merging to pick the one with some params
and use that plus mark params :autodetected when merged from code.

But the actual merging of params is still done in Param class
because of the complicated types merging logic.

Additionally refactored the param warnings creation to separate
method.