add a test for empty strings

sorted('') == sorted('')
>>> True
This doesn't conform to the problem spec.

arrays_strings/permutation/permutation_solution.ipynb

@@ -101,6 +101,8 @@
     "    def is_permutation(self, str1, str2):\n",
     "        if str1 is None or str2 is None:\n",
     "            return False\n",
+    "        if str1 == ''   or str2 == '':\n",
+    "            return False\n",
     "        return sorted(str1) == sorted(str2)"
    ]
   },