Find the first character in a string that appears only once. Return null if no such character exists or if the input is empty/null.
Examples:
Input: str = "salesforce"
Output: "a"
Explanation: 's' appears twice, 'a' appears once, so 'a' is the first non-repeating character
Input: str = "aabbcc"
Output: null
Explanation: All characters appear twice, so return null
Input: str = "cloudcloud"
Output: null
Explanation: Every character repeats, so return null
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