Solved: The problem was that I was indicating an empty path for the files to include during the fx:jar process.
Just to clarify, what was confusing was that the error was pointing to line 19. But it just tells that the whole block of XML (starting at line 19) is where the error is, and in fact the problem was in one of the internal sections, in this specific case, the problem was:
<fileset dir="${build.classes.dir}" />
I just changed the parameter of dir to where I really host my .class files, and problem solved.