Count the number of unique characters in a given string. Ignore case when counting (so 'A' and 'a' are considered the same character). Return 0 for null or empty strings.
This challenge tests your understanding of Sets for tracking unique values and string manipulation for case-insensitive operations.
Examples:
Input: input = "hello"
Output: 4
Explanation: The unique characters are h, e, l, o (l appears twice but is only counted once)
Input: input = "Hello World"
Output: 8
Explanation: Case-insensitive unique characters are h, e, l, o, w, r, d, space (8 unique characters)
Input: input = "aaa"
Output: 1
Explanation: Only one unique character: a
Input: input = null
Output: 0
Explanation: Null input returns 0
Input: input = ""
Output: 0
Explanation: Empty string returns 0
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