Merge two lists of contact names and return a sorted list with no duplicates. Comparison should be case-insensitive. If both lists are null or empty, return an empty list.

Examples:

List<String> list1 = new List<String>{ 'Alice', 'Bob' };
List<String> list2 = new List<String>{ 'Charlie', 'alice' };
List<String> merged = mergeContactLists(list1, list2);
// Results: ['Alice', 'Bob', 'Charlie']
// Explanation: Case-insensitive deduplication and sorting

List<String> list1 = new List<String>{ 'David' };
List<String> list2 = new List<String>{ 'Eve', 'Frank' };
List<String> merged = mergeContactLists(list1, list2);
// Results: ['David', 'Eve', 'Frank']
// Explanation: Merge and sort without duplicates
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