Improve test generators

[ErikSchierboom]

• Render null as null without requiring wrapping in an unescaped values
• Use Assert.Null if using Assert.Equal(null, x)
• Don't render quote character in test method/class names

[robkeim]

Nice work @jpreese getting these issues fixed! Are there any other issues we've discovered so far that we should be tracking here, or should we go ahead and close this issue?

[ErikSchierboom]

I think we can close.

[jpreese]

Yeah it depends if we want to keep this as a rolling issue and keep adding to it, or just close it out and open PRs as we find stuff. I'm a fan of the latter since I don't really like issues that just never have a clear finish line.

I have a couple other ideas, but I was just going to do PRs for them, so I'll go ahead and close this.