Fix oslo_versionedobjects warnings

This patch set fixes warnings regarding invalid UUIDs and
static_root.

Change-Id: Icb0bbca9c05ee97ea9947a31db5e87b7837e42d0

watcher/objects/fields.py

@@ -33,7 +33,14 @@ NonNegativeIntegerField = fields.NonNegativeIntegerField
 ObjectField = fields.ObjectField
 StringField = fields.StringField
 UnspecifiedDefault = fields.UnspecifiedDefault
-UUIDField = fields.UUIDField
+
+
+class UUIDField(fields.UUIDField):
+    def coerce(self, obj, attr, value):
+        if value is None or value == "":
+            return self._null(obj, attr)
+        else:
+            return self._type.coerce(obj, attr, value)
 
 
 class Numeric(fields.FieldType):