Given a list of integers nums and an integer k, return the total number of contiguous subarrays whose elements sum to exactly k. The list may contain negative numbers. Return 0 for null or empty input.
Examples:
Input: nums = [1, 1, 1], k = 2
Output: 2
Explanation: [1,1] at indices 0-1 and [1,1] at indices 1-2 both sum to 2
Input: nums = [1, 2, 3], k = 3
Output: 2
Explanation: [3] at index 2 and [1,2] at indices 0-1 both sum to 3
Input: nums = [1, -1, 0], k = 0
Output: 3
Explanation: [1,-1] sums to 0, [1,-1,0] sums to 0, and [0] sums to 0, giving 3 subarrays total
Input: nums = [1, 2, 3], k = 7
Output: 0
Explanation: No contiguous subarray sums to 7
Input: nums = null, k = 5
Output: 0
Explanation: Null input returns 0
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