HOT get_attr returning empty string if there is no value breaks validation

[Randall Burt]

The get_attr HOT intrinsic function returns an empty string in cases where it cannot reconcile a value. However, for LIST properties (and probably MAP as well), this results in validation failure during initial stack-create if you map a LIST property's value to an attribute of another resource that returns a list. This seems to have cropped up when get_attr was modified to allow traversing complex attribute values.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/77713

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/77713
Committed: https://git.openstack.org/cgit/openstack/heat/commit/?id=f5fcfb45cfc79405e5cc5628047720ac0ce418a8
Submitter: Jenkins
Branch: master

commit f5fcfb45cfc79405e5cc5628047720ac0ce418a8
Author: Randall Burt <email address hidden>
Date: Mon Mar 3 16:18:17 2014 -0600

    Return None when get_attr cannot resolve a value

    To prevent validation failures prior to resource creation, get_attr
    returns None if the attribute or the sub-value cannot be resolved.
    This returns this function to consistency with prior behavior and allows
    non-string attribute values to be resolved to their proper default
    "empty" data types during initial template validation.

    Change-Id: I0e02174c43be65a647358a2047da8213635f85d8
    Closes-Bug: 1287397