fix: handle empty struct in CompareSchemasVisitor.struct()

[waterWang]

Fixes #17650

Problem

CompareSchemasVisitor.struct() computes its result as:

Result result = fields.stream().reduce(Result::merge).orElse(Result.SCHEMA_UPDATE_NEEDED);

For a struct with zero fields, the reduce is empty, so orElse(SCHEMA_UPDATE_NEEDED) always fires. Consequently any schema containing an empty struct (e.g. derived from an Avro union whose branch is a zero-field marker record) can never compare SAME with any table schema — including a table created from that exact schema.

In the Dynamic Sink this is fatal: TableUpdater.findOrCreateSchema sees SCHEMA_UPDATE_NEEDED, applies EvolveSchemaVisitor (a no-op here), commits, re-compares — still SCHEMA_UPDATE_NEEDED — and TableMetadataCache.schema() caches and returns NOT_FOUND, whose resolvedTableSchema() is null.

Fix

Change orElse(Result.SCHEMA_UPDATE_NEEDED) to orElse(Result.SAME) — when both structs have zero fields, they are semantically identical.

Changes

• flink/v1.20/flink/src/main/java/.../CompareSchemasVisitor.java
• flink/v2.0/flink/src/main/java/.../CompareSchemasVisitor.java
• flink/v2.1/flink/src/main/java/.../CompareSchemasVisitor.java

All three versions had the same bug (identical content).

[ebyhr]

Can we add a regression test, or do you think it's overkill?

[uros-b]

+1 with @ebyhr for adding regression testing. Issue #17650 supplies a ready repro, right @waterWang?