Given a list of integers and a non-negative integer k, return the number of unique index pairs (i, j) where i is less than j and the absolute difference between the values at those indices equals k. Duplicate values count each distinct index pair separately. Return 0 for null input, empty input, or null k.
Examples:
Input: nums = [1, 2, 3, 4, 5], k = 2
Output: 3
Explanation: The pairs are (1,3), (2,4), and (3,5)
Input: nums = [1, 1, 1, 1], k = 0
Output: 6
Explanation: Every pair of indices has equal values, and there are 4 choose 2 equals 6 pairs
Input: nums = [1, 2, 3], k = 10
Output: 0
Explanation: No pair has a difference of 10
Input: nums = [], k = 2
Output: 0
Explanation: Empty list has no pairs
Input: nums = null, k = 1
Output: 0
Explanation: Null input returns 0
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