input: Write a python function to merge two arrays into one.
  assert merge_arrays([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert merge_arrays([10, 20], [30, 40, 50]) == [10, 20, 30, 40, 50]
  assert merge_arrays([], [1, 2, 3]) == [1, 2, 3]
output: def merge_arrays(arr1, arr2):
      return arr1 + arr2