[flang] Merge pull request flang-compiler/f18#822 from flang-compiler/pmk-nullptr

Remove explicit comparisons to `nullptr` and most usage of `has_value()`

Original-commit: flang-compiler/f18@fdb351ca2afb0d71028785da4687113343e11f54
Reviewed-on: https://github.com/flang-compiler/f18/pull/822