Semantically, testing that method with positive object or null input parameter would not make sense but if you look from that function's behaviour standpoint, both input parameters creates a different use-case regardless of the output.
I would handle this through custom exceptions like ValidationException and except to throw for a specific set of input parameter. Infact, Java has built-in Objects class consisting of static validation methods for input parameters.
Although Optional type may indicate to have two possible results(i.e., null/non-null result) yet it is supportive only from convenience's standpoint; i.e, to avoid null and handling alternate situations during development but does not imply any different meaning from the function's behaviour standpoint because, Ideally, you should never be in a situation where for the same input parameter, you'd have to test two different behaviours just because you have Optional return type.