Avoid using len(x) to check if x is empty

This cases are using `len()` to check if collection has items.
As collections have a boolean representation too, directly check
for true / false.

Change-Id: I1e552db99fbe6d33ad43bb710af924261dcebb46

keystone/common/validation/validators.py

@@ -43,7 +43,7 @@ class SchemaValidator(object):
         except jsonschema.ValidationError as ex:
             # NOTE: For whole OpenStack message consistency, this error
             # message has been written in a format consistent with WSME.
-            if len(ex.path) > 0:
+            if ex.path:
                 # NOTE(lbragstad): Here we could think about using iter_errors
                 # as a method of providing invalid parameters back to the
                 # user.