Is it permissible not to perform ghusl if one doubts having had a wet dream and did not check whether semen was discharged or not, and then found no trace of semen on his clothes after istinja', despite feeling wetness?

Ghusl is only obligatory upon certain conviction that semen has been discharged. As long as you are not certain of that, ghusl is not obligatory upon you, and what you did is correct. If you are certain that something was discharged but doubt whether it is semen or something else, you have a choice, and you may assign it the ruling of whatever you wish.