Can't retrieve null value via record indexing using field name from record

[philrz]

tl;dr

I expected the middle value returned to be the null but instead got error("missing").

$ echo '{field1:"UserID",field2:"SearchPhrase",field3:"State",r:{UserID:1,SearchPhrase:null,State:"CA"}}' | super -Z -c 'yield r[field1],r[field2],r[field3]' -
1
error("missing")
"CA"

Details

Repro is with super commit 66b20d0.

The retrieval works fine if I use static strings as the field names in the record index expression.

$ super -version
Version: v1.18.0-227-g66b20d0f

$ echo '{field1:"UserID",field2:"SearchPhrase",field3:"State",r:{UserID:1,SearchPhrase:null,State:"CA"}}' | super -Z -c 'yield r["UserID"],r["SearchPhrase"],r["State"]' -
1
null
"CA"

I looked back at the history of this one. The last time it worked correctly was back at commit 1338193.

$ zq -version
Version: v0.33.0-147-g1338193f

$ echo '{field1:"UserID",field2:"SearchPhrase",field3:"State",r:{UserID:1,SearchPhrase:null,State:"CA"}}' | zq -z 'yield r[field1],r[field2],r[field3]' -
1
null
"CA"

When the changes associated with #3532 arrived in the next commit, the null value stopped appearing entirely.

$ zq -version
Version: v0.33.0-148-g7d7184b7

$ echo '{field1:"UserID",field2:"SearchPhrase",field3:"State",r:{UserID:1,SearchPhrase:null,State:"CA"}}' | zq -z 'yield r[field1],r[field2],r[field3]' -
1
"CA"

Thing stayed like that for a while, then with the changes in #3569 it started coming back as error("missing") as we see through today.

$ zq -version
Version: v0.33.0-172-g4fb52305

$ echo '{field1:"UserID",field2:"SearchPhrase",field3:"State",r:{UserID:1,SearchPhrase:null,State:"CA"}}' | zq -z 'yield r[field1],r[field2],r[field3]' -
1
error("missing")
"CA"