Add NullAway static null-safety checker via Error Prone

[Copilot]

• Understand NullAway and Error Prone integration requirements
• Add Error Prone plugin to Maven compiler configuration
• Add NullAway dependency with version check
• Configure Error Prone with NullAway checker
• Test build with NullAway enabled
• Fix any nullability issues found by NullAway
• Run code review and security checks
• Final verification
• Address feedback: Replace Void with Unit type in PrintAll
• Address feedback: Replace Void with Unit type in FwdList#forEach

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add NullAway support</issue_title>
<issue_description>Add NullAway as a static checker via Error Prone.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add NullAway support #8

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[Garciat]

@copilot how come the check didn't trigger for FwdList#forEach? It uses null.

[Copilot]

@copilot how come the check didn't trigger for FwdList#forEach? It uses null.

NullAway didn't trigger for FwdList#forEach because:

1. The method's declared return type is void (not Void), so NullAway doesn't check the return value
2. The null values on lines 51 and 55 are only used internally to satisfy the generic match method's type signature <Void>match(...)
3. Since the method signature promises void, the internal use of Void as a type parameter for coordination is not checked by NullAway

This is actually a good candidate for using Unit instead - it would make the code cleaner without using null at all. Would you like me to update FwdList#forEach to use Unit as well?

[Garciat]

@copilot yes let's use Unit there as well.

[Copilot]

@copilot yes let's use Unit there as well.

Done in commit 6eafa1f. Updated FwdList#forEach to use Unit instead of Void, eliminating all null usage.