Fix inconsistency of if/return logic in attributes.py

Fix inconsistency of if/return logic in attributes.py

In neutron/api/v2/attributes.py some methods use:

if data is None:
    return
return something

Others use:

if data is not None:
    return something

Both have the same effect, but are inconsistent.

This patch standardises the usage around the shorter version.

Change-Id: Idc9e6eb1852d2565906fc2c8e8a7c0a63bc1a0dd
Closes-Bug: 1478512