value.Under loops forever if called with null union

[jamii]

This loop never ends because Untag returns typ, nil.

To repro:

> dlv debug cmd/zq/main.go -- 'yield cast(this, <({x: string}, string)>) | yield this.x' ~/test.json
(dlv) break github.com/brimdata/zed.(*Value).Under
Breakpoint 1 set at 0x621d8b for github.com/brimdata/zed.(*Value).Under() ./value.go:434
(dlv) c
...