Add private constructor and null validation to CollectionUtils.

[qkrtkdwns3410]

Summary

Improve CollectionUtils utility class with defensive programming practices.

Changes

1. Add private constructor

• Prevent instantiation of utility class
• Follow Java best practices for utility classes

2. Add null validation with Assert.notNull()

• Enforce the contract documented in Javadoc (@param list must not be null)
• Fail fast with clear error message instead of NullPointerException

3. Add unit tests for null validation

• getFirstShouldRejectNullList()
• getLastShouldRejectNullList()

Why these changes?

Before	After
No instantiation prevention	Private constructor
Silent NPE on null input	Clear IllegalArgumentException
No test for null case	Test coverage added

[mp911de]

CollectionUtils is internal API, we don't need to add additional complexity and guards since this code is used internally only.