Find the length of the longest substring without repeating characters in a given string. Return the maximum length as an Integer. For null or empty strings, return 0.
Examples:
Input: s = 'abcabcbb'
Output: 3
Explanation: The longest substring without repeating characters is 'abc', with length 3.
Input: s = 'bbbbb'
Output: 1
Explanation: The longest substring is 'b', with length 1.
Input: s = 'pwwkew'
Output: 3
Explanation: The longest substring is 'wke', with length 3. Note that 'pwke' is not valid because it is not a substring.
Input: s = ''
Output: 0
Explanation: Empty string has no characters.
Input: s = null
Output: 0
Explanation: Null string returns 0.
Input: s = 'abcdef'
Output: 6
Explanation: The entire string has no repeating characters.
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