Optional label have been integrated into the `TrapLabel` type by
using the undefined id, which is now returned by `fetchLabel` on
`nullptr` entities.

At trap emission time however, if the field was expected not to be
optional, a `missing_properties` trap entry is emitted.

The test was inspired by locally running the query against files in
https://github.com/apple/swift/tree/main/test/Parse

An interesting byproduct was finding a problematic `assert` in the
Swift headers. An incomplete `FallthroughStmt` was asserting on having
a destination. I did not find any other sensible way of getting rid of
the crash when running in debug mode than to patch the header.