If it is not an empty string or an array, then it gives back an URL-encoded-based 64 string. If it is empty, or an array, then it gives back an empty string.
{{ "apple"|trackable_link_encode }}-->"YWxtYQ%3D%3D"
{{ ""|trackable_link_encode }}-->""
{{ []|trackable_link_encode }}-->""
{{ ["apple", "lemon"] | trackable_link_encode }}-->""