Calculate the Least Common Multiple (LCM) of two positive integers. The LCM is the smallest positive integer that is divisible by both numbers. Use the relationship LCM(a,b) = (a * b) / GCD(a,b) where GCD is the Greatest Common Divisor. Return null for invalid inputs (negative numbers, zero, or null).
Examples:
Input: a = 4, b = 6
Output: 12
Explanation: 12 is the smallest number divisible by both 4 and 6
Input: a = 3, b = 5
Output: 15
Explanation: 3 and 5 are coprime, so LCM is their product
Input: a = 12, b = 18
Output: 36
Explanation: GCD(12, 18) = 6, so LCM = (12 * 18) / 6 = 36
Input: a = 7, b = 7
Output: 7
Explanation: LCM of identical numbers is the number itself
Input: a = 0, b = 5
Output: null
Explanation: Zero is invalid input
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