We don't catch when null is assigned to a non-nulllable (inferred) variable

[erikcorry]

main:
  foo := non_nullable
  while foo:
    foo = nullable

non_nullable -> int:
  return 1

nullable -> int?:
  return null

This runs, and doesn't catch the place where we assign null to foo. It does warn though, because foo is non-nullable so the while is always true.