Research isn't about taking a statement and proving that it is true. That would assume that it is, in fact, true without the research being done. Research is about determining if something is true or not.
But there are two possibilities here. One, which occurs in statistical studies, is a result that shows insufficient evidence that the theory is true. Another, that can occur there or in, say mathematics, is a result that shows that the theory is clearly wrong.
The first of those is a failed experiment. It may say little about the model. But the latter is a definite result. "We thought this might be true and learned that it isn't".
Note that in statistical testing, some fraction of the samples used will give false results. If the study is well designed, then you can predict what that fraction is, say .01.
But showing that a model or a theory is clearly wrong results in a good thesis and yes, you say "This theory is wrong!!!"
A failed statistical theory may, however, require more work, or the advisor and committee may accept it, provided that the design was good and everything else done carefully and in good faith.
Knowledge is knowledge and it is good, whether positive or negative.
In fact, many published studies are later proved false because the investigators started out with a preconceived notion of the truth and did what they could to "make it so." That isn't research. Research is stepping into the unknown to try to make it known.