Compute the digital root of a non-negative integer. The digital root is obtained by repeatedly summing the digits of the number until only a single digit remains. For example, the digital root of 9875 is 2 because 9 + 8 + 7 + 5 = 29, and 2 + 9 = 11, and 1 + 1 = 2. Return null when the input is null or negative.
Examples:
Input: n = 7
Output: 7
Explanation: A single digit is its own digital root.
Input: n = 38
Output: 2
Explanation: 3 + 8 = 11, then 1 + 1 = 2.
Input: n = 9999
Output: 9
Explanation: 9 + 9 + 9 + 9 = 36, then 3 + 6 = 9.
Input: n = 0
Output: 0
Explanation: The digital root of 0 is 0.
Input: n = -5
Output: null
Explanation: Negative input returns null.
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