[Feature] TableWrite skip null fields check, when RowKind is DELETE

### Search before asking

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

### Motivation

When deleting a row from PrimaryKey Table, we can set the RowKind type to RowKind.DELETE.

Currently, the TableWriteImpl class will always invoke checkNullability method, ensure all fileds comply with null constraint,

We suggest only verify primary keys null constraint if RowKind is DELETE, then when delete a row, only need provide primary keys.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by locating TableWriteImpl and its checkNullability call, then trace how RowKind.DELETE records are written and how primary-key tables enforce null constraints. Confirm the change by checking that delete rows require only non-null primary keys while non-delete rows still validate all fields.

Written by the indexing model from the issue text.