fix a major bug in entity validation

ChangeLog

 ChangeLog for yams 
 ------------------
+	
+ --
+    * fix a major bug in entity validation
+    * refactor RelationFileReader to ease customization
 
 2006-09-11  --  0.6.1
     * consider meta when a relation type is implicitly created

schema.py

@@ -370,7 +370,7 @@ class EntitySchema(ERSchema):
                 continue
             # skip other constraint if value is None and None is allowed
             if value is None and not required:
-                break
+                continue
             if not aschema.check_value(value):
                 errors[rtype] = 'incorrect value %r for type %s' % (value,
                                                                     aschema.type)