Allow for missing parameter map

[dkozma]

Description

This fixes an issue where we get an error if an action assertion doesn't contain a parameter map. According to the C2PA 1.0 spec, parameters is optional and may not exist.

Motivation and Context

We should be able to handle assets that conform to the spec without errors occurring.

How Has This Been Tested?

Tested with Verify on sample images.

Types of changes

• [x] 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:

• [x] I have signed the Adobe Open Source CLA.
• [x] 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.

[dkozma]

Replacing with #14 due to #13.