Fixes crash when attribute exists with value 'None'

In the hypothetical situation an object happened to have the "scons" attribute with a non iterable value, like None, which could hipothetically happen when importing content to a Plone site with this product installed -  the former line 93 would retrieve the non iterable, though improbable, and line 96 would generate an exquisite exception.

Not that I am saying that this could happen  - but we better be prepared.