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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