fixed yaml error

lib/spack/spack/architecture.py

@@ -79,6 +79,8 @@ def to_dict(self):
 
     @staticmethod
     def from_dict(d):
+        if d is None:
+            return None
         target = Target.__new__(Target)
         target.name = d['name']
         target.module_name = d['module_name']

lib/spack/spack/spec.py

@@ -651,10 +651,13 @@ def to_node_dict(self):
         d = {
             'variants' : dict(
                 (name,v.enabled) for name, v in self.variants.items()),
-            'arch' : self.architecture.to_dict(),
             'dependencies' : dict((d, self.dependencies[d].dag_hash())
                                   for d in sorted(self.dependencies))
         }
+        if self.architecture:
+            d['arch'] = self.architecture.to_dict()
+        else:
+            d['arch'] = None
         if self.compiler:
             d.update(self.compiler.to_dict())
         else: