There are patterns that we declare empty attribute in every testcase objects. For example:
https://searchfox.org/mozilla-central/rev/91ed81b76015e49ebd55a6de5df2b034456c15e8/browser/extensions/formautofill/test/unit/test_getFormInputDetails.js#32-34

We can make the code a little bit cleaner by assuming it is empty if the attribute is not defined. An example of how we can remove redundant code can be found in this patch