Node form context elements are not rendered/loaded properly

I think this is the D7 version of #965148: node add/edit and taxonomy pages are not rendered when using site template.

The site template fails to render or probably even load panel elements building on the form contexts – all you get is an "array" text string.

Easiest way to reproduce:

* Enable site template. Make sure the page content pane is included.
* Add a variant for node/%node/edit. No content is necessary. (No actual edit form needs to be present.)
* Add a new node.

-> You get an empty page saying array.

This error only appears for panel variants – not for HTTP response codes.

I know Panels/Page manager have problems loading node form contexts. It works if the context is loaded using path arguments, but not contexts added with relations or manually selected IDs. Maybe it is related (but Panels everywhere fails managing even the cases where Page manager works).