fix: Do not repeat processing of icon if it exists.

[bstopp]

Description

If there is exists an image tag in icon ref, skip processing that icon.

Related Issue

Fix: #128

Motivation and Context

Prevent accidental repeated processing of elements for icons.

How Has This Been Tested?

Added unit tests.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.