So ofcourse I know that the sentence is perfectly fine on it's own without the addition of who can do the action. It just seems to me that it is omitted because it is obvious and not because it is not actually there. Maybe I am wrong. But if you had to put a word there, it would be in the dative right? because of the infinitive verb.

But yeh is their no filler word to put in there, in dative, that would result in the same sentence. It is just hard for me learning to have gaps and assuming what they mean, but I guess that's how this language works.