add CombinationSum and test (#6725)

* add CombinationSum and test

* Format array and list literals in CombinationSumTest

* Fix sorting comparator in CombinationSumTest

* Refactor CombinationSum for better handling and clarity

Updated combinationSum method to handle null or empty candidates and improved variable naming for clarity.

* Fix sorting comparator in CombinationSumTest

* Update CombinationSumTest.java

* Fix sorting comparator for list of integers

* Fix formatting issues in CombinationSum class

* Change CombinationSum class to final

* Refactor norm method to accept Iterable instead of List

* Remove unnecessary whitespace in CombinationSumTest