More fixes for cypress relationships.

[mshima]

Related to #16617, #16504 and #14103.

Deploy fixes for:

• Fixes react many-to-one relationships
  Api call were passing flatten relationships values eg:

userId: 1
user: {id: 1}

• Reactive doesn't allows update operation for entities without fields or relationships.
  Fix: return the original entity without executing the sql operation.

Deploy workarounds for:

• Reactive + MariaDB doesn't allow null Instant fields (Not applicable to our frontend, it's never null).
  Workaround: never pass null to Instant fields.
• Reactive + PostgresDB fails to insert/update entities without data.
  Workaround: always pass at least 1 field.

---

Please make sure the below checklist is followed for Pull Requests.

• All continuous integration tests are green
• Tests are added where necessary
• The JDL part is updated if necessary
• jhipster-online is updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed

When you are still working on the PR, consider converting it to Draft (below reviewers) and adding skip-ci label, you can still see CI build result at your branch.