debuginfo: Handle LineNumberNode with `file=nothing` (#61938)

We currently assume `file` is a symbol when converting to
`LegacyLineInfoNode`, which is then passed to
the `DebugInfo` constructor giving us `def=nothing` out of nowhere. This
     change cleans this up slightly.

Disallowing `file=nothing` in LineNumberNode (or aggressively converting
to
     empty Symbol) might be a nicer way of doing things in the future.