Given a list of integers, return a new list with all even numbers appearing first in their original relative order, followed by all odd numbers in their original relative order. Return null for null input and an empty list for empty input.
Examples:
Input: nums = [1, 2, 3, 4, 5]
Output: [2, 4, 1, 3, 5]
Explanation: Evens [2, 4] come first, then odds [1, 3, 5]
Input: nums = [3, 1, 5]
Output: [3, 1, 5]
Explanation: No even numbers, so the list is returned with odds only
Input: nums = [2, 4, 6]
Output: [2, 4, 6]
Explanation: All numbers are even, so the list is returned unchanged
Input: nums = []
Output: []
Explanation: Empty list returns empty list
Input: nums = [7]
Output: [7]
Explanation: Single odd number
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