Calculate the number of complete months between two dates. The start date is inclusive. If the start date is after the end date, return a negative number. If either date is null, return null.

Examples:

Input: startDate = Date.newInstance(2024, 1, 15), endDate = Date.newInstance(2024, 4, 15) Output: 3 Explanation: Three complete months from January 15 to April 15 Input: startDate = Date.newInstance(2024, 1, 1), endDate = Date.newInstance(2024, 1, 31) Output: 0 Explanation: Less than one complete month within same month Input: startDate = Date.newInstance(2023, 6, 1), endDate = Date.newInstance(2024, 6, 1) Output: 12 Explanation: Exactly one year equals 12 months Input: startDate = Date.newInstance(2024, 6, 1), endDate = Date.newInstance(2024, 1, 1) Output: -5 Explanation: When start date is after end date, return negative months
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