Yeah I could see that you meant to tell that since for the language {a} the only valid string is a itself the statement would be trivially true isn't it?
Oh that's very right.I think we should consider all of the possible options to prove that the P.L for the DFA is 3 isn't it?I think that's a tedious task.Maybe that's why every one take P.L to be greater than or equal to the number of states in DFA isn't it?