[INLONG-8173][Sort] Fix the NPE problem when adding new columns

[lordcheng10]

Prepare a Pull Request

(Change the title refer to the following example)

• Title Example: [INLONG-XYZ][Component] Title of the pull request

(The following XYZ should be replaced by the actual GitHub Issue number)

• Fixes [Bug][Sort] Fix the NPE problem when adding new columns #8173

Motivation

When Column#position may be null, the doAddColumn method may throw NPE:

inlong/inlong-sort/sort-flink/sort-flink-v1.13/sort-connectors/iceberg/src/main/java/org/apache/inlong/sort/iceberg/schema/IcebergSchemaChangeHelper.java

Lines 134 to 136 in a38bab2

	column.getPosition().getPositionType() == PositionType.FIRST ? TableChange.ColumnPosition.first()
	: TableChange.ColumnPosition.after(column.getName());
	TableChange.AddColumn addColumn = new TableChange.AddColumn(new String[]{column.getName()},

Modifications

When Column#position is null, append to the last column by default.

Verifying this change

(Please pick either of the following options)

• This change is a trivial rework/code cleanup without any test coverage.

• This change is already covered by existing tests, such as:
  (please describe tests)

• This change added tests and can be verified as follows:

  (example:)

  • Added integration tests for end-to-end deployment with large payloads (10MB)
  • Extended integration test for recovery after broker failure

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a follow-up issue for adding the documentation

[lordcheng10]

@gong @EMsnap @yunqingmoswu PTAL,thanks!