core: add is_empty method to func_adapter

Since before_replace triggers have different semantics for returned nil
value and no return values, we need to distinguish between these cases.
That is why func_adapter_is_null does not return true when no more
values left anymore - new method func_adapter_is_empty is introduced
for this purpose.

Along the way, remove description of func_adapter_lua_is_bool function -
it is copied description of is_null function, which was here by mistake.

Part of #8657

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