       "name": "stdout",
       "output_type": "stream",
       "text": [
        "Empty input\n",
        "One element\n",
        "Two or more elements\n",
        "Success: test_merge_sort\n"
 
</s> remove     "    if len(data) < 2:\n",
 </s> add     "    if data is None or len(data) < 2:\n", </s> remove    "name": "python2"
 </s> add    "name": "python3" </s> remove    "name": "python2"
 </s> add    "name": "python3"
      "None input\n",
       "name": "stdout",
       "output_type": "stream",
       "text": [
      "None input\n",
        "Empty input\n",
        "One element\n",
        "Two or more elements\n",
        "Success: test_merge_sort\n"
keep keep add keep keep keep keep
 Polish merge sort challenge and solution

Add None input test case.  Update space complexity discussion. 
sorting_searching/merge_sort/merge_sort_solution.ipynb