Given a positive integer n, return the length of the Collatz sequence starting from n. The Collatz sequence is generated by repeatedly applying the rule: if the current value is even, divide it by 2; if odd, multiply by 3 and add 1. The sequence terminates when the value reaches 1. The length includes both the starting value and the terminal 1. Return null when the input is null, zero, or negative. Use Long arithmetic internally because intermediate values may exceed the Integer range.
Examples:
Input: n = 1
Output: 1
Explanation: The sequence is just [1].
Input: n = 6
Output: 9
Explanation: 6, 3, 10, 5, 16, 8, 4, 2, 1 contains 9 values.
Input: n = 7
Output: 17
Explanation: 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1.
Input: n = 0
Output: null
Explanation: Zero is not a valid starting value.
Input: n = null
Output: null
Explanation: Null input returns null.
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