More Comprehensive Edge Case Testing

[Bhavesh9908]

This PR improves the ArrayFillTest class in the org.apache.commons.lang3 package by addressing null safety, adding edge case coverage, and refactoring redundant test code for better readability and maintainability.

Changes:
Refactor Using @ParameterizedTest:

Combined similar test cases into a single parameterized test method using @ParameterizedTest and @MethodSource, reducing redundancy in tests for different data types (e.g., byte[], char[], double[], String[]).
Null Safety Enhancements:

Updated tests for null arrays to explicitly check that ArrayFill.fill() returns null when a null array is passed.
Example: testFillByteArrayNull, testFillCharArrayNull, etc., now include assertions to verify null handling.
Improved Readability:

Replaced numeric literals for char arrays with more readable character literals (e.g., 'A' instead of 1) in testFillCharArray.
Edge Case Coverage:

Added new test cases for:
Empty Arrays: Verifying behavior when filling an empty array.
Arrays with Default Values: Ensuring ArrayFill.fill() correctly handles arrays already populated with default values.
Object Arrays with null Values: Added a test to ensure ArrayFill.fill() can handle filling object arrays with null values.
New Test Methods:

testFillEmptyArray: Ensures the method works correctly for empty arrays.
testFillObjectArrayWithNull: Tests behavior when filling an object array with null values.
testFillArrayWithDefaultValues: Verifies behavior when arrays already have default values.

[garydgregory]

Hello @Bhavesh9908
The code coverage for ArrayFill is already at 100%:
https://commons.apache.org/proper/commons-lang/jacoco/org.apache.commons.lang3/ArrayFill.html#dn-a

Feel free to increase code coverage elsewhere.