PY-60054 - clear "pydev_message" and always set result of evaluated breakpoint expression

[rendner]

https://youtrack.jetbrains.com/issue/PY-60054/Pycharm-debugger-evaluate-expression-incorrect-results-when-value-is-None

This PR fixes the problem with the stale data logged by the evaluation breakpoints (for None and empty strings).

I'm not sure where the best point is to clear a previous set pydev_message, maybe this place is not the best one.