[flink] Set default parallelism for sink in cases where AQE not supported

[yunfengzhou-hub]

Purpose

This PR provides default parallelism configuration for sink operators, in cases where undefined/adaptive parallelism has not been supported.

Tests

Existing UTs are modified to verify that the cases mentioned above can function correctly now.

API and Format

No API or format is changed

Documentation

No documentation needed. The existing unsupported cases had not been documented, so no modification needed for documents.