omit placeholders for external identifiers

[olli-gold]

References

• Fixes Placeholders in ISSN field #490

Description

This PR solves the problem, that a placeholder may be submitted to ORCID.

Instructions for Reviewers

The problem occurs, when you want to push an item to ORCID.

List of changes in this PR:

• Omit an external ID if it's just a placeholder

Checklist

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & integration tests). Exceptions may be made if previously agreed upon.
• My PR passes Checkstyle validation based on the Code Style Guide.
• My PR includes Javadoc for all new (or modified) public methods and classes. It also includes Javadoc for large or complex private methods.
• My PR passes all tests and includes new/updated Unit or Integration Tests based on the Code Testing Guide.
• If my PR includes new libraries/dependencies (in any pom.xml), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
• If my PR modifies REST API endpoints, I've opened a separate REST Contract PR related to this change.
• If my PR includes new configurations, I've provided basic technical documentation in the PR itself.
• If my PR fixes an issue ticket, I've linked them together.

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