fix: synchronize angular image empty attributes on updates

[pawelphilipczyk-cloudinary]

Pull request for cloudinary/frontend-frameworks

For which package is this PR?

• @cloudinary/ng

What does this PR solve?

• Follow up to Angular sync image attributes #235
• Synchronises empty image attributes including allowing to set empty string and remove attributes that have nullish values
• See this comment for more information: https://cloudinary.atlassian.net/browse/SNI-8216?focusedCommentId=355626

Final checklist

• Implementation is aligned to Spec.
• Tests - Add proper tests to the added code.
• Relates to a github issue (link to issue).