Add assertions about non nullability

[greg0ire]

While working on #10301, I found that the improvements I made resulted in a lot of Psalm errors. A lot of them look like they should also exist with doctrine/lexer 1, so here is a PR adding assertions, that way it will make it easier to assess the impact of future developments in that area of the code.

This also makes me think we might want a better Lexer API with separate methods where we don't expect to obtain null. It seems to be the case most of the time.