How should I respond to "no research position available" from a cold email?
There’s no need to respond.

An (uninvited) question has been asked and (kindly) answered. Answering required the professor’s time. There’s no need to follow up, because that will require more of the professor’s time. 




  Obviously, [the answer] is not positive.


I don’t know whether you mean “the professor answered that the lab has no open research positions” or the professor does not see you as a promising candidate or similar. Regarding the latter, you should not be concerned. The professor probably receives many such requests and simply doesn’t have the time to provide lengthy replies. (That’s why I recommend not to respond.) 




  Why would a professor waste their time with evaluating the suitability of a candidate if there are no positions available


The professor might not have even read the email, let alone evaluated the suitability of the candidate. Indeed, the professor might be using canned responses, which can be sent almost as quickly as an email can be deleted/archived and are perhaps better than non-responses.