Task Dependency Resolver

You are given an array of unique tasks (which can be strings or numbers) and an array of dependency pairs. Each dependency is represented as a two-element array [a, b] indicating that task a must be completed before task b.

Write a function that returns a valid ordering of tasks that satisfies all the dependencies. If no valid ordering exists (i.e., there's a cyclic dependency), return an empty array.

Details:

• The tasks array contains all the tasks in the system, and some tasks might not have any dependencies.
• The dependencies array contains pairs [a, b] which means task a must come before task b.
• If multiple valid orderings exist, returning any one of them is acceptable.

Example:

Input:
tasks: ["A", "B", "C", "D"]
dependencies: [["A", "B"], ["B", "C"], ["D", "C"]]

Possible Output:
["A", "D", "B", "C"] or ["D", "A", "B", "C"]

Implement the solution with attention to time and space efficiency. You may use any programming language of your choice.