The first statement checks for the existence of `var`, the second checks that it has a value other than `null` and will throw and error if `var` does not exist.

If you're not sure whether a variable exists, you should check if it exists first then check for its value to be safe.

You could combine both statements like so `{% if var is defined and var is not null %}` or use `empty, not empty` if you're feeling fancy: )