Actually, on reading this test case title... I think this would be a break.

By default, "allow empty" is `false`, and is used to determine if a required input is allowed to have an empty value. If you mark an element as not required, the assumption is that we allow an empty value -- this is why the "allow empty" flag defaults to `false`. If we change that, we break a ton of assumptions -- which you can observe in the CollectionInputFilter tests.

So, the question I have, then is: should calling `setRequired(false)` automatically set the "allow empty" flag to `true`? That would allow this to work -- but it also means that order of operations becomes critical -- you would be required to set the "allow empty" flag *after* setting the required flag if you wanted to set it to `false`.