Handle unions in toZedScript

[philrz]

Fixes #3098, which has more background.

The video below shows the change having the intended benefit on the original repro shown in #3098, and also with this simpler input file foo.ndjson:

{"_path": "http", "foo": "bar"}
{"_path": 1, "foo": "baz"}
{"_path": true, "foo": "bonk"}

Demo.mp4

[jameskerr]

Yup, that looks right to me.