Fixes to UNPIVOT normalization and empty struct typing

[RCHowell]

Relevant Tests

• Unpivot AT Alias Normalization
  • equiv wildcard steps struct
    • PERMISSIVE
    • LEGACY
  • equiv path expression with wildcard steps
    • PERMISSIVE
    • LEGACY
  • unpivotMissing
    • PERMISSIVE
    • LEGACY
  • unpivotEmptyStruct
    • PERMISSIVE
    • LEGACY
  • unpivotStructWithMissingField
  • selectFromScalarAndAtUnpivotWildCardOverScalar
• UNPIVOT Empty
  • pathUnpivotEmptyStruct1
  • pathUnpivotEmptyStruct2
  • pathUnpivotEmptyStruct3
• Unable to resolve IS_STRUCT
  • equiv attribute value pair unpivot missing

• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? [YES/NO]
  Yes

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report-Cross Engine

	Base (legacy)	eval	+/-
% Passing	92.54%	85.97%	-6.57%
✅ Passing	5384	5002	-382
❌ Failing	434	816	382
🔶 Ignored	0	0	0
Total Tests	5818	5818	0
Number passing in both: 4829

Number failing in both: 261

Number passing in legacy engine but fail in eval engine: 555

Number failing in legacy engine but pass in eval engine: 173
⁉️ CONFORMANCE REPORT REGRESSION DETECTED ⁉️
The complete list can be found in GitHub CI summary, either from Step Summary or in the Artifact.
173 test(s) were failing in legacy but now pass in eval. Before merging, confirm they are intended to pass.
The complete list can be found in GitHub CI summary, either from Step Summary or in the Artifact.

Conformance comparison report-Cross Commit-LEGACY

	Base (41c00fd)	8c03cdc	+/-
% Passing	92.51%	92.54%	0.03%
✅ Passing	5382	5384	2
❌ Failing	436	434	-2
🔶 Ignored	0	0	0
Total Tests	5818	5818	0
Number passing in both: 5382

Number failing in both: 434

Number passing in Base (41c00fd) but now fail: 0

Number failing in Base (41c00fd) but now pass: 2
The following test(s) were previously failing but now pass. Before merging, confirm they are intended to pass:

Click here to see

• MYSQL_SELECT_29, compileOption: PERMISSIVE

• MYSQL_SELECT_29, compileOption: LEGACY

Conformance comparison report-Cross Commit-EVAL

	Base (41c00fd)	8c03cdc	+/-
% Passing	85.70%	85.97%	0.28%
✅ Passing	4986	5002	16
❌ Failing	832	816	-16
🔶 Ignored	0	0	0
Total Tests	5818	5818	0
Number passing in both: 4986

Number failing in both: 816

Number passing in Base (41c00fd) but now fail: 0

Number failing in Base (41c00fd) but now pass: 16
16 test(s) were previously failing but now pass. Before merging, confirm they are intended to pass
The complete list can be found in GitHub CI summary, either from Step Summary or in the Artifact.