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

[Illia Khudoshyn]

`len()` is used to check if collection (e.g., a dict, list, set, etc.) has items. As collections have a boolean representation too, it is better to directly check for true / false.

rally/common/utils.py
rally/task/utils.py
rally/task/validation.py
tests/unit/doc/test_specs.py

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/276996

[Julien Danjou (jdanjou)]

I don't think spamming multiple projects with false bugs is a good idea. Please stop.

[Sean McGinnis (sean-mcginnis)]

I have to agree with Julien.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/276886
Committed: https://git.openstack.org/cgit/openstack/rally/commit/?id=efd641caf0ad0bba0c6ad0993a5b65d08078966d
Submitter: Jenkins
Branch: master

commit efd641caf0ad0bba0c6ad0993a5b65d08078966d
Author: Steve Wilkerson <email address hidden>
Date: Fri Feb 5 13:38:51 2016 -0600

    Use booleans to check for empty lists

    Changed from checking for empty collections with
    len(list) to checking the boolean value of the list
    instead

    Closes-Bug: #1538518
    Change-Id: Ib601a83b8b6e19ab78690f8ca2834e7ef622cb9b

[OpenStack Infra (hudson-openstack)]

Change abandoned by Ren Qiaowei (<email address hidden>) on branch: master
Review: https://review.openstack.org/276996