Generate the Collatz sequence starting from a given positive integer. The sequence follows these rules: if the current number is even, divide it by 2; if it is odd, multiply it by 3 and add 1. The sequence ends when it reaches 1 (include 1 in the result). Return null for null input and an empty list for values less than 1.
Examples:
Input: n = 6
Output: [6, 3, 10, 5, 16, 8, 4, 2, 1]
Explanation: 6 is even so 6/2=3, 3 is odd so 3*3+1=10, 10/2=5, 5*3+1=16, 16/2=8, 8/2=4, 4/2=2, 2/2=1
Input: n = 1
Output: [1]
Explanation: The sequence starts and ends at 1
Input: n = 8
Output: [8, 4, 2, 1]
Explanation: 8 is even so 8/2=4, 4/2=2, 2/2=1
Input: n = null
Output: null
Explanation: Null input returns null
Input: n = 0
Output: []
Explanation: Values less than 1 return an empty list
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