That's actually up for philosophical debate. For instance, if I were to tell you there might be a fly in your house somewhere, you wouldn't consider it unlikely even in the absence of evidence, whereas if I said there was a kangaroo, it would be quite agreeable to accept the null hypothesis because the alternative is very unlikely.

IOW, if we accepted absence of evidence as evidence of absence, there'd be no scientific research. Just because most research thus far has been crappy doesn't mean learning styles are bullshit. It intuitively seems, at least, that some people are more visual than others, and some are more capable of abstract thought than others, etc.