Absolutely not. It's still possible to write a dissertation of a decent standard whilst having to reject your null hypotheses. It's not really about what your results show, more about your awareness of why your results may go against the existing literature. Including factors such as confounding variables and thoroughly investigating the limitations of your study will show an understanding of why you didn't get the results you expected. Including this, and also what further research could do to control for confounding variables, will help you receive better marks.