reject function giving error when all the items in the list are null

reject() and the other looping functions are legacy, and don't handle corner cases very well.  Almost all of them can be safely replaced by a!forEach() (sometimes wrapped in a!flatten()).  FWIW, the only legacy function I know of which can't be replaced thus, is reduce().

a!flatten(a!forEach(local!myList, if(a!isNullOrEmpty(fv!item), {}, fv!item)))

reject() and the other looping functions are legacy, and don't handle corner cases very well.  Almost all of them can be safely replaced by a!forEach() (sometimes wrapped in a!flatten()).  FWIW, the only legacy function I know of which can't be replaced thus, is reduce().

a!flatten(a!forEach(local!myList, if(a!isNullOrEmpty(fv!item), {}, fv!item)))

I strongly agree with Mike. Ever since I realized that {} does not get returned in output, I have stopped using reject(). Please bear in mind that if you replace {} with anything in the above expression, then it will not be removed.