Have to give a talk soon but I no longer believe in my research
Do a practice talk with your advisor before going to the conference. Do it early. It will be a good time to discuss how the talk should be organized in order to  highlight the context of your measurement in the whole project and its relative importance. 

It is common for a student working on a small piece of problem (and not necessarily looking at its entire context), to go back and forth between feeling A) the piece is obvious or unimportant; to B) the piece is pretty cool after all; then back to A again. 

At worst - if it seems as if your piece really is not so relevant, then I expect he will help you organize the talk so that it still engages the audience. (Not a bad skill to practice. And, if you don't pull it off - at least in my field, if the topic of the talk seems unimportant to the audience, the bad smell lingers longer on the professor/advisor than on the student).

(Of course, also in my subfield, giving a talk at a conference or workshop usually means that we are giving a talk about work-in-progress still unpublished. So strategies may be different if there is a required paper involved for every talk).

But more likely, with working on the practice talk and the discussion with the advisor - you will develop more of an appreciation of how that small piece of work fits and may even develop an appreciation of its other subtleties that interest the professor. In a situation as you describe, I place my bets on the professor instinct on the topic.