[Schema] Fix catalog getTable failing for bucketed append tables created with Paimon 0.7

[ArnavBalyan]

Purpose

• Paimon 0.7 allowed creating fixed bucket append tables without a bucket-key.
• The bucket validation added in 1.0 runs on every table read and fails for the legacy tables which dont have a key.
• The deserializer already handles this with bucket=1 for old schemas, but validation does not check this.
• Skip this check for older versions.

Tests

• UT Added
• CI

[ArnavBalyan]

cc @JingsongLi thanks!