Form State should have empty-array 'input' value set in admin form

Note, this might be related to issue #549952, but that's on 6.x, so I haven't been able to retest on that branch.

The NodeFormCol admin menu attempts to prebuild the node form to see what elements are available to reorder, but it fails to set an empty 'input' value in form_state. This causes some modules (most notably date) to throw errors because 'input' is always set, but is oftentimes an empty array.

For example, in Date the Select field calles drupal_array_get_nested_value($form_state['input']) and will throw a non-fatal (but still fatal!) error if the input is not at least an empty array.

Attached is a one-liner patch to fix this.