fix(VolumeViewport): added null actorEntry check in VolumeViewport.getCurrentImageId

[jbocce]

Context

The was a null exception thrown in VolumeViewport.getCurrentImageId when no actor entry was present.

Changes & Results

Added a null check.

Testing

Attempt to call VolumeViewport.getCurrentImageId right after the viewport element is enabled. It should not throw and exception.

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

• I have run the yarn build:update-api to update the API documentation, and have
  committed the changes to this PR. (Read more here https://www.cornerstonejs.org/docs/contribute/update-api)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• OS: Windows 11
• Node version: 19.18.1
• Browser: Chrome 113.0.5672.127

[netlify]

✅ Deploy Preview for cornerstone-3d-docs ready!

Name	Link
🔨 Latest commit	28f157d
🔍 Latest deploy log	https://app.netlify.com/sites/cornerstone-3d-docs/deploys/64665eae985f8400087039aa
😎 Deploy Preview	https://deploy-preview-618--cornerstone-3d-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.