api: Don't do version check if nothing required

There's no reason to inspect the API version requested (and reject
things if unset) if we don't have version constraints on the API
resource. This lets us add schema decorators to unversioned API
resources (i.e. the "versions" API at '/').

Change-Id: Ib4d4585e3e4664eba8cd3daa47b4349eb2bfed74
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

nova/api/openstack/api_version_request.py

@@ -389,10 +389,10 @@ class APIVersionRequest(object):
         If self is null then raise ValueError
         """
 
-        if self.is_null():
-            raise ValueError
         if max_version.is_null() and min_version.is_null():
             return True
+        elif self.is_null():
+            raise ValueError
         elif max_version.is_null():
             return min_version <= self
         elif min_version.is_null():