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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