Implement READ_STRING_FIELD_NULL serializable read function.

[wenchaozhang-123]

When there is "" string, we need also to palloc memory in OidAssignment.

Authored-by: Zhang Wenchao zwcpostgres@gmail.com

fix #ISSUE_Number

---

Change logs

Describe your change clearly, including what problem is being solved or what feature is being added.

If it has some breaking backward or forward compatibility, please clary.

Why are the changes needed?

Describe why the changes are necessary.

Does this PR introduce any user-facing change?

If yes, please clarify the previous behavior and the change this PR proposes.

How was this patch tested?

Please detail how the changes were tested, including manual tests and any relevant unit or integration tests.

Contributor's Checklist

Here are some reminders and checklists before/when submitting your pull request, please check them:

• [ ] Make sure your Pull Request has a clear title and commit message. You can take git-commit template as a reference.
• [ ] Sign the Contributor License Agreement as prompted for your first-time contribution(One-time setup).
• [ ] Learn the coding contribution guide, including our code conventions, workflow and more.
• [ ] List your communication in the GitHub Issues or Discussions (if has or needed).
• [ ] Document changes.
• [ ] Add tests for the change
• [ ] Pass make installcheck
• [ ] Pass make -C src/test installcheck-cbdb-parallel
• [ ] Feel free to request cloudberrydb/dev team for review and approval when your PR is ready🥳