Fix null analysis warnings in test code (#245)

- Remove unused variable nullByteHash in ModifiableByteTest
- Add explicit 'this' qualifier to field accesses in ModifiableIntegerTest

These changes address static analysis warnings without changing functionality.
The warning about methods being "potentially static" is a false positive
for JUnit 5 test methods which cannot be static.

Co-authored-by: Developer <[email protected]>

Author: ic0ns