gracefully handle absent href attribute when parsing links

[asmundstavdahl]

Description

Previously, an 'Invalid parameters passed' error would be emitted when parsing an "a" element without an "href" attribute. This fix lets href-less links through. Thanks to @santerref of #1562 for this fix.

Fixes #1562

Checklist:

• [ ] I have run composer run-script check --timeout=0 and no errors were reported
  • There were errors, but they are not caused by the changes in this PR and should be fixed separately IMHO
• [x] The new code is covered by unit tests (check build/coverage for coverage report)
• [ ] I have updated the documentation to describe the changes
  • I could not find a suitable place to document it, and also this is a fix for existing functionality and should not need to be documented.