Null means nothing, zero. In the context of scientific articles a null result means that the difference is zero. What difference? Well it depends. It could be difference between doing something and not doing it. The difference between before and after some intervention. Or perhaps the difference between two different actions.

In this case the difference between before and after raising the minimum wage.

Furthermore, the thing with a null result is that it's always dependant on how sensitive your study is. A null result is always of the form "we can't rule out that it's 0". If the study is powerful then a null result will rule out a large difference, but there is always the possibility that there is a difference too small to detect.

Null means something doesn't exist and proving something doesn't exist can at times especially in certain sciences be as valuable as proving something does exist. Especially if it stops the rest of the field chasing the pot of gold at the end of the rainbow.