odds are the target was neutral when it didn't work, to make sure it would cast against anything not friendly then you should use [nohelp]. That will work unless you have some sort of addon affecting macro interpretation. The empty set [] basically makes a macro with parameters cast as though none of the parameters existed as long as nothing before it evaluates to true. So for it to work as a failsafe you would have to have a valid target selected as though you had simply dragged the spell out of your spell book onto the action bar and clicked it.

Regardless of what's happening with the [] or [harm] [target=target] is always useless as it simply sets your target to your target...

When working with castsequences and /click macros, make sure all the individual components work apart from the /click. It'll save you a lot of headache trying to hunt down a problem.