Given a Date value, return the last day of that month. Return null for null input.
Examples:
Input: d = Date.newInstance(2024, 1, 15)
Output: Date.newInstance(2024, 1, 31)
Explanation: January has 31 days
Input: d = Date.newInstance(2024, 2, 10)
Output: Date.newInstance(2024, 2, 29)
Explanation: 2024 is a leap year, so February has 29 days
Input: d = Date.newInstance(2023, 2, 5)
Output: Date.newInstance(2023, 2, 28)
Explanation: 2023 is not a leap year, so February has 28 days
Input: d = Date.newInstance(2024, 4, 1)
Output: Date.newInstance(2024, 4, 30)
Explanation: April has 30 days
Input: d = null
Output: null
Explanation: Null input returns null
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