input: Write a python function to merge two sorted arrays into one sorted array.
  assert merge_arrays([1, 3, 5], [2, 4, 6]) == [1, 2, 3, 4, 5, 6]
  assert merge_arrays([10, 20, 30], [5, 15, 25]) == [5, 10, 15, 20, 25, 30]
  assert merge_arrays([1, 2, 3], []) == [1, 2, 3]
  assert merge_arrays([], [7, 8, 9]) == [7, 8, 9]
output: def merge_arrays(arr1, arr2):
      merged_array = []
      i, j = 0, 0
      while i < len(arr1) and j < len(arr2):
          if arr1[i] < arr2[j]:
              merged_array.append(arr1[i])
              i += 1
          else:
              merged_array.append(arr2[j])
              j += 1
      while i < len(arr1):
          merged_array.append(arr1[i])
          i += 1
      while j < len(arr2):
          merged_array.append(arr2[j])
          j += 1
      return merged_array