image/appc: fix volume conversion and add tests

util/dict_wrappers.py

@@ -59,8 +59,16 @@ class JSONPathDict(object):
   def __getitem__(self, path):
     return self.get(path)
 
+  def __iter__(self):
+    return self._object.itervalues()
+
+  def iterkeys(self):
+    return self._object.iterkeys()
+
   def get(self, path, not_found_handler=None):
     """ Returns the value found at the given path. Path is a json-path expression. """
+    if self._object == {} or self._object is None:
+      return None
     jsonpath_expr = parse(path)
     matches = jsonpath_expr.find(self._object)
     if not matches:
@@ -77,4 +85,3 @@ class JSONPathDict(object):
 
   def keys(self):
     return self._object.keys()
-