fix: add null input validation to AlternativeStringArrange.arrange() (#7425)

* fix: add null input validation to AlternativeStringArrange.arrange()

The arrange() method previously threw a NullPointerException when either
input string was null. This change explicitly validates the inputs and
throws IllegalArgumentException with a clear message, matching the
fail-fast pattern used by other utility classes in this package (e.g.
HammingDistance).

- Add null guard at the start of arrange()
- Update Javadoc with @throws and contract notes
- Add parameterized test covering all three null-input combinations

* fix: remove unused JUnit @Test import (Checkstyle violation)