Don't render empty edit (configuration) forms.

At the moment the way the Edit (configuration) forms for blocks works is that if the defining module implements the homebox_block_edit_form hook then it will render a form, regardless of whether the modules implementation of the hook has logic the only returns forms for certain deltas, this means if a multiple deltas are in use on Homebox and only one of them is actually defining a form, that both of them will have a form rendered regardless, the second being just a button.

Following patch adds a check before rending the form to see if the form is actually populated or not.