E_NOTICE if block is themed without an argument

If block content is created using a theme() call that does not take an argument, as in

$block['content'] = theme('amazon_store_search_block');

Then you get the E_NOTICE message:

Notice: Undefined index: render element in contextual_preprocess() (line 82 of modules/contextual/contextual.module).

The attached patch just checks for this situation and avoids it. The code already dealt with the situation of not having found an element.