Given a string, return a new string where characters at even indices (0, 2, 4, ...) are uppercase and characters at odd indices (1, 3, 5, ...) are lowercase. Return null for null input and return an empty string for empty input.
Examples:
Input: str = 'hello'
Output: 'HeLlO'
Explanation: H at index 0 (even), e at index 1 (odd), L at index 2 (even), l at index 3 (odd), O at index 4 (even)
Input: str = 'apex'
Output: 'ApEx'
Explanation: A at index 0 (even), p at index 1 (odd), E at index 2 (even), x at index 3 (odd)
Input: str = 'salesforce'
Output: 'SaLeSfOrCe'
Input: str = null
Output: null
Input: str = ''
Output: ''
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