Update reverse string challenge (#113)

Update input check logic to check for an empty but not None entry.

arrays_strings/reverse_string/reverse_string_solution.ipynb

@@ -97,8 +97,8 @@
     "class ReverseString(object):\n",
     "\n",
     "    def reverse(self, chars):\n",
-    "        if chars is None:\n",
-    "            return None\n",
+    "        if chars is None or not chars:\n",
+    "            return chars\n",
     "        size = len(chars)\n",
     "        for i in range(size//2):\n",
     "            chars[i], chars[size-1-i] = \\\n",
@@ -126,13 +126,13 @@
     "class ReverseStringAlt(object):\n",
     "\n",
     "    def reverse_string_alt(string):\n",
-    "        if string is None:\n",
-    "            return None\n",
+    "        if string is None or not string:\n",
+    "            return string\n",
     "        return string[::-1]\n",
     "\n",
     "    def reverse_string_alt2(string):\n",
-    "        if string is None:\n",
-    "            return None\n",
+    "        if string is None or not string:\n",
+    "            return string\n",
     "        return ''.join(reversed(string))"
    ]
   },