Given two strings, interleave them by alternating characters from each string. Start with the first character of the first string, then the first character of the second string, then the second character of the first string, and so on. If one string is longer than the other, append the remaining characters to the end. Treat null inputs as empty strings.
Examples:
Input: s1 = 'abc', s2 = 'xyz'
Output: 'axbycz'
Explanation: Characters alternate: a, x, b, y, c, z
Input: s1 = 'abcd', s2 = 'xy'
Output: 'axbycd'
Explanation: After interleaving, remaining characters from s1 are appended
Input: s1 = '', s2 = 'hello'
Output: 'hello'
Explanation: Empty first string means only second string characters remain
Input: s1 = null, s2 = null
Output: ''
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