go/vt/wrangler: fix nilness issues and unused variable

[mdlayher]

Description

Fixes:

✔ ~/src/github.com/vitessio/vitess/go/vt/wrangler [main|✔] 
16:02 $ nilness ./...
/home/matt/src/github.com/vitessio/vitess/go/vt/wrangler/materializer.go:321:14: tautological condition: non-nil != nil
/home/matt/src/github.com/vitessio/vitess/go/vt/wrangler/traffic_switcher.go:1923:10: impossible condition: nil != nil
/home/matt/src/github.com/vitessio/vitess/go/vt/wrangler/vexec.go:664:9: impossible condition: nil != nil

The results map I've removed was immediately shadowed on old line 725. I marked the query string as const as well just to add a bit more safety.

Related Issue(s)

Updates #14684.

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

n/a

[vitess-bot]

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