Not filled, white voids near boundaries of polar plot

I have wrote about a problem using matplotlib polar cotourf last week, in the issues of the GitHub repository, which didn’t get any feedback or answer yet. Because of no answering in the issues, I post it in stack overflow, too, as a bounty question, which didn’t get any answer, too. I would be grateful if anyone who know any solution or … to answer in one of these two links.