The attributes form element must not be NULL

Problem/Motivation

It was reported over in Authcache (#2885086: Check to make sure element is an array before checking to see if it needs to be flagged), that the ubercart attributes module adds a form element 'attributes' => NULL when in the add-to-cart form for products which do not have any product attributes.

This is a violation of the form API contract, since elements (i.e. those array entries whose key does not start with a #-character) are expected to contain arrays. As a result any code which uses the core element_children function on the add-to-cart form might produce notices.

Example of an affected form structure:

Array
(
    [nid] => Array
    [qty] => Array
    [actions] => Array
    [node] => Array
    [attributes] => NULL
    [#form_id] => uc_product_add_to_cart_form_39
    [#type] => form
    [#build_id] => form-pQth...
    [form_build_id] => Array
    [#token] => uc_product_add_to_cart_form_39
    [form_token] => Array
    [form_id] => Array
    [#id] => uc-product-add-to-cart-form-39
    [#method] => post
    [#action] => /page-path
    [#theme_wrappers] => Array
    [#tree] => NULL
    [#parents] => Array
    [#validate] => Array
    [#submit] => Array
    [#theme] => Array
    [#after_build] => Array
)

Proposed resolution

Do not add the attributes element if there are no product-attributes in the first place.

Remaining tasks

User interface changes

None.

API changes

None.

Data model changes

None.