isNotFirst method
Return true if the given item
is NOT the same (by identity) as the first iterable item.
If this iterable is empty, always return null.
This is useful for non-indexed loops where you need to know when you don't have the first
item. For example:
for (student in students) {
if (children.isNotFirst(student) result.add(Divider());
result.add(Text(student.name));
}
Implementation
bool isNotFirst(T item) => !isFirst(item);