Fix customTextRenderer not working on documents without marked content

[MattL75]

Fixes #1530

@wojtekmaj I think this is a good approach for now. I don't think there can be a mix of marked and unmarked content (at least not any use cases that make sense). So one or the other should work. Added a test for this case as well. Feel free to reformat :)

I confirmed with the document in the ticket, the highlighting occurs in the test project.

[MattL75]

1530

[corystevens]

I can confirm that this fixes my issue.

[wojtekmaj]

Thanks!