Implement a function that converts all strings in a list to uppercase. If an element in the list is null, leave it as null. If the input list is null, return an empty list.
Examples:
Input: ["hello", "world", null, "apex"]
Output: ["HELLO", "WORLD", null, "APEX"]
Explanation: Each string is converted to uppercase, nulls remain as null
Input: null
Output: []
Explanation: Input list is null, so we return an empty list
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