Handle null input in Maskify

[krotname]

Motivation

• The Maskify.maskify method dereferenced its str parameter via str.length() and threw a NullPointerException when called with null, so a minimal robustness fix is needed.

Description

• Add a null guard to Maskify.maskify so it returns null when called with a null argument and otherwise preserves the original masking behavior.
• Add a regression test nullInputShouldRemainNull in MaskifyTest that asserts maskify(null) returns null.
• Changes are minimal and limited to src/main/java/kyu7/Maskify.java and src/test/java/kyu7/MaskifyTest.java to avoid altering existing functionality.

Testing

• Ran mvn -q -Dtest=MaskifyTest test and the test suite targeting the modified test passed successfully.
• Ran mvn -q test for the full test suite and it completed successfully.

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Codex Task

[krotname]

Superseded by the validated combined merge in #115: #115