Given a list of lists of integers (nested list), flatten it into a single list containing all integers in order. For example, [[1, 2], [3, 4], [5]] should become [1, 2, 3, 4, 5].
Return an empty list for null or empty input.
Examples:
Input: nestedList = [[1, 2], [3, 4], [5]]
Output: [1, 2, 3, 4, 5]
Explanation: All integers from sublists combined into single list
Input: nestedList = [[10], [20, 30], [40, 50, 60]]
Output: [10, 20, 30, 40, 50, 60]
Explanation: Flatten three sublists into one list
Input: nestedList = null
Output: []
Explanation: Null input returns empty list
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