[Bug] Paimon CDC not working in Dynamic Bucket Mode

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Paimon version

The latest code in master branch on Apr 22, commit id = 80040f69d593056e6b8daf18099f87360266e5e7

### Compute Engine

Flink

### Minimal reproduce step

Use `CREATE DATABASE AS` clause in Flink SQL to synchronize an existing database to a Paimon catalog in tests.

### What doesn't meet your expectations?

The following error is thrown in the Flink job

```
Caused by: java.lang.AssertionError
at org.apache.paimon.table.sink.KeyAndBucketExtractor.bucket(KeyAndBucketExtractor.java:53)
at org.apache.paimon.table.sink.FixedBucketRowKeyExtractor.bucket(FixedBucketRowKeyExtractor.java:70)
at org.apache.paimon.flink.sink.RowDataKeyAndBucketExtractor.bucket(RowDataKeyAndBucketExtractor.java:50)
...
```

### Anything else?

I found that the problem is caused by the following invocation process

CdcDynamicBucketSink -> CdcRecordKeyAndBucketExtractor -> KeyAndBucketExtractor.bucket

`CdcDynamicBucketSink` is used when numBuckets = -1, but `KeyAndBucketExtractor.bucket` requires that numBuckets > 0, or an AssertionError would be thrown.

From this point of view, similar problem should also exist for postpone bucket mode, where numBuckets = -2.

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Reproduce the failure with the Flink SQL CREATE DATABASE AS flow, then trace CdcDynamicBucketSink through CdcRecordKeyAndBucketExtractor to KeyAndBucketExtractor.bucket. Review the related FixedBucketRowKeyExtractor and RowDataKeyAndBucketExtractor classes, including the -1 dynamic and -2 postpone bucket cases. Done means the reproduction no longer throws AssertionError for those modes.

Written by the indexing model from the issue text.