evalv3: struct.MinFields fails too early under evalv3

[rogpeppe]

What version of CUE are you using (cue version)?

67f9dfa9

Does this issue reproduce with the latest stable release?

Yes (v0.10.0)

What did you do?

exec cue vet x.cue
env CUE_EXPERIMENT=evalv3
exec cue vet x.cue

-- x.cue --
import "struct"

#foo: struct.MinFields(1) & {
	run?: string
}

What did you expect to see?

Passing test.

What did you see instead?

> exec cue vet x.cue
> env CUE_EXPERIMENT=evalv3
> exec cue vet x.cue
[stderr]
#foo: invalid value {run?:string} (does not satisfy struct.MinFields(1)): len(fields) < MinFields(1) (0 < 1):
    ./x.cue:3:7
    ./x.cue:3:24
[exit status 1]
FAIL: /tmp/y.txtar:3: unexpected command failure

The vet should not fail with a definition that could have more fields in it later.