fix: NullReferenceException during ImportReferences in - #1410

NetworkBehaviourILPP (#1378)

In some cases, for example when assembly is System.Colletions.Immutable, null value is passed to .Add() method causing NullReferenceException
during the build. Simply guarding against it solves the issue.

Co-authored with @JadenH

PR Checklist

• Have you added a backport label (if needed)? For example, the type:backport-release-* label. After you backport the PR, the label changes to stat:backported-release-*.
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR or a link to the documenation repo PR if this is a manual update.

Changelog

com.unity.netcode.gameobjects

• Added: The package whose Changelog should be added to should be in the header. Delete the changelog section entirely if it's not needed.
• Fixed: If you update multiple packages, create a new section with a new header for the other package.
• Removed/Deprecated/Changed: Each bullet should be prefixed with Added, Fixed, Removed, Deprecated, or Changed to indicate where the entry should go.

Testing and Documentation

• No tests have been added.
• Includes unit tests.
• Includes integration tests.
• No documentation changes or additions were necessary.
• Includes documentation for previously-undocumented public API entry points.
• Includes edits to existing public API documentation.