I believe that's already the case in Asciidoctor.js 3.0.

Asciidoctor > 2.0.18 is released

It will actually change the behavior, in Asciidoctor.js 3.0 we return undefined but when building against Asciidoctor > 2.0.18 we return ''. Should Iwe return undefined or ''?

I think we should return undefined. I'm going to update core to return empty string when that's what it should be, but there are times when it should be undefined (block macro).

but there are times when it should be undefined (block macro).

Could you please provide a simple reproduction case where it returns Opal.nil?

The original description provides an example case when it currently returns Opal.nil (but should return undefined).

Here's another case:

console.dir(Asciidoctor.load('image::test.png[]').getBlocks()[0].getContent())

We know now that this is only happening in the 2.2.x line. The 3.0.x line seems to be doing the right thing.