config: fix config.Dict with no value type

recipe_engine/config.py

Index: recipe_engine/config.py
diff --git a/recipe_engine/config.py b/recipe_engine/config.py
index dd473a1cf78a16b8f29a0d89944330da4d131000..e0048adba924c3108db4e55ff7a33b0c4e9c5f00 100644
--- a/recipe_engine/config.py
+++ b/recipe_engine/config.py
@@ -620,8 +620,9 @@ class Dict(ConfigBase, collections.MutableMapping):
     if isinstance(val, Dict):
       val = val.data
     typeAssert(val, collections.Mapping)
-    for v in val.itervalues():
-      typeAssert(v, self.value_type)
+    if self.value_type:
+      for v in val.itervalues():
+        typeAssert(v, self.value_type)
     self.data = val
 
   def as_jsonish(self, _include_hidden=None):