Calculate the Greatest Common Divisor (GCD) of two positive integers using the Euclidean algorithm. The GCD is the largest number that divides both numbers without leaving a remainder. Return null if either number is null or not positive.

Examples:

Input: a = 48, b = 18 Output: 6 Explanation: 6 is the largest number that divides both 48 and 18 Input: a = 17, b = 5 Output: 1 Explanation: Coprime numbers share no common divisors except 1 Input: a = 100, b = 25 Output: 25 Explanation: 25 divides evenly into 100 Input: a = 0, b = 5 Output: null Explanation: Zero is not positive, return null Input: a = null, b = 5 Output: null Explanation: Null input, return null
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