go/vt/vtgate: fix nilness issues

[mdlayher]

Description

Resolve a handful of nilness issues in the vtgate packages.

✔ ~/src/github.com/planetscale/vitess/go/vt/vtgate [main|✔] 
12:08 $ nilness ./...
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/evalengine/api_coerce.go:51:10: impossible condition: nil != nil
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/engine/routing.go:276:16: tautological condition: nil == nil
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/engine/vindex_lookup.go:234:9: impossible condition: nil != nil
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/planbuilder/operators/query_planning.go:79:10: impossible condition: nil != nil
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/planbuilder/operators/query_planning.go:225:49: impossible condition: nil != nil
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/planbuilder/operators/vindex.go:157:10: impossible condition: nil != nil
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/planbuilder/operator_transformers.go:254:9: impossible condition: nil != nil
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/planbuilder/operator_transformers.go:434:9: impossible condition: nil != nil

There is one I did not fix because it may be an actual false positive due to the loop enclosing it:

✔ ~/src/github.com/planetscale/vitess/go/vt/vtgate [mdl-vtgate-nilness|✔] 
12:08 $ nilness ./...
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/engine/routing.go:276:16: tautological condition: nil == nil

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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[ajm188]

thank you!

[ajm188]

huh

[mdlayher]

Yeah, the diff is silly but I figured it made more sense to hoist the error return that wrapped the error vs keeping the bare return.

[frouioui]

Thank you!!