return 404 error on missing action

moto/core/responses.py

@@ -272,6 +272,9 @@ class BaseResponse(_TemplateEnvironmentMixin):
                     headers['status'] = str(headers['status'])
                 return status, headers, body
 
+        if not action:
+            return 404, headers, ''
+
         raise NotImplementedError(
             "The {0} action has not been implemented".format(action))
 

moto/core/utils.py

@@ -18,6 +18,8 @@ def camelcase_to_underscores(argument):
     python underscore variable like the_new_attribute'''
     result = ''
     prev_char_title = True
+    if not argument:
+        return argument
     for index, char in enumerate(argument):
         try:
             next_char_title = argument[index + 1].istitle()