Java: switching over nullable values may trigger NPE

[GreyCat]

Follow up for #300 — original issue mentioned case of enum being null and then being switched over. The problem is actually much deeper and, currently, it's pretty hard to solve without major reworking of expression type derivation to include nullability tracking.

Java allows switching over String or boxed integers (like Integer, Long), which can be null due to:

• if failing
• type switching match failed without default case
• something else?

A very simple repro:

seq:
  - id: always_null
    type: u1
    if: false
  - id: trigger_npe
    type:
      switch-on: always_null
      cases:
        1: one
        _: two

This issue will acts as a reminder, may be we'll return to it one day.