Given two lists of strings, return a list containing only the elements that appear in both lists. The result should contain each common element only once and be sorted alphabetically. Handle null inputs by returning an empty list.
Examples:
Input: list1 = ['apple', 'banana', 'cherry'], list2 = ['banana', 'cherry', 'date']
Output: ['banana', 'cherry']
Explanation: Only banana and cherry appear in both lists, sorted alphabetically
Input: list1 = ['a', 'b', 'c'], list2 = ['d', 'e', 'f']
Output: []
Explanation: No common elements between the two lists
Input: list1 = null, list2 = ['a', 'b']
Output: []
Explanation: Null input returns empty list
Input: list1 = ['x', 'y'], list2 = null
Output: []
Explanation: Null input returns empty list
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