Reporting negative results/failure of experiments in thesis/papers
I agree  with you but suggest that you follow your advisor's advice about your dissertation. To a fairly large extent you are limited by the standards of your advisor until you finish your degree. Even when their advice isn't optimal.
Research is a search for truth, not for results. Negative results can be just as important as positive ones in the search for truth and should, in principle, be reported so that, just as you say, others don't follow blind alleys.
But too many people, I think have the opposite view that a negative result is a failure in some sense, even when it contributes to knowledge. Your advisor may be one of these.
But the purpose of the dissertation is to get it approved and get the degree. After that, the standards you follow can be (more or less) your own. So, a tactical retreat here may lead to a strategic victory. Your dissertation won't be your last, nor, hopefully, your best work.