Given a credit card number as a string, mask all characters except the last 4 by replacing them with the '#' character. Return null for null input and an empty string for empty input. If the string has 4 or fewer characters, return it unchanged.
Examples:
Input: cardNumber = '4111111111111111'
Output: '############1111'
Explanation: All characters except the last 4 are replaced with '#'
Input: cardNumber = '12345'
Output: '#2345'
Explanation: Only the first character is masked, last 4 are preserved
Input: cardNumber = '1234'
Output: '1234'
Explanation: String has 4 or fewer characters, return unchanged
Input: cardNumber = null
Output: null
Explanation: Null input returns null
Input: cardNumber = ''
Output: ''
Explanation: Empty string returns empty string
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