No, there isn't a filter that lets you find questions without comments. This question is a feature request for a comment filter, but it doesn't have an answer, and I don't expect that we'll get a comment filter any time soon. AFAIK, the internal search engine ignores comments, when I want to find old comments I use a
site: search in Google.
I don't think there'd be a lot of utility for a search function that finds questions with zero comments. A question with no votes, no comments, and no answers is considered to be abandoned, and the system automatically deletes such questions, as Jeff Atwood explained here. If an unanswered question has comments, it will survive in the system a little longer.
If you're searching for juicy unanswered questions to answer, don't worry about the comments, even if the question appears to be answered in the comments. As Catija mentions, answers in comments are not proper answers, and you are improving the site by converting such comments into actual answers that people can vote on. Of course, if you use (or mention) other people's ideas in your answer you should give them some credit.