Fix equals method on simple identifiers

[javierestevez]

Pull Request Description

Small PR to fix the equals method for simple identifiers (the value can be null but it's expected to not be null inside the equals method).

Type of Pull Request

• New feature/Enhancement - non-breaking change which adds functionality
• Bug fix
• Breaking change (Require Major version change?)

Type of Change

• OpenPeppol AS2/AS4 specification
• OpenPeppol Spring/Fall release
• Oxalis software change or enhancement
• CEF change

Pull Request Checklist:

• My code follows the style guidelines of this project
• [NA] I have commented my code, particularly in hard-to-understand areas. But did not added unnecessary annotation/comment say @author name etc
• I have checked my code for variable and method name and corrected grammar/spelling mistakes if any
• [NA] I have made corresponding changes to the documentation where needed
• [NA] My changes generate no new/additional warnings
• My change is not breaking or creating conflict with associated dependencies
• I have performed a self-review of my own code
• [NA] I ran mvn clean install before commit and all tests run successfully
• [NA] I conducted basic QA to assure all features are working fine
• [NA] My pull request generate no conflicts with master branch
• [NA] I requested code review from other team members