fix(null-safety): Add null checks in various segmentation related code.

[jbocce]

Context

null checks found while testing OHIF PR OHIF/Viewers#5701

Changes & Results

Various null checks added.

Testing

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

System:
OS: Windows 11 10.0.26200
CPU: (20) x64 12th Gen Intel(R) Core(TM) i7-12700H
Memory: 4.90 GB / 31.68 GB
Binaries:
Node: 22.17.0 - C:\Users\joebo\AppData\Local\fnm_multishells\14456_1767626745599\node.EXE
Yarn: 1.22.22 - C:\Program Files (x86)\Yarn\bin\yarn.CMD
npm: 10.1.0 - C:\Users\joebo\AppData\Local\fnm_multishells\14456_1767626745599\npm.CMD
bun: 1.2.23 - C:\Users\joebo.bun\bin\bun.EXE
Browsers:
Chrome: 143.0.7499.170