fix(segmentation): Fix issue exposed by interpolationContourSegmentation example

[jbocce]

Context

Run the https://www.cornerstonejs.org/live-examples/interpolationcontoursegmentation example. Notice that an exception is thrown when clicking on an image to apply a segmentation to it.

Using git bisect I was able to determine that #2231 caused this issue.

Changes & Results

Initialize the active segment index to the first available segment if none is currently set.

Testing

Run the example. It should behave as expected.

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.)

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: 23.9.0
• "Browser: Chrome 140.0.7339.81