DataObjects that return an empty PreviewURL result in 404 in preview panel

[GuySartorelli]

It is perfectly valid for a CMSPreviewable object to return null or an empty string from the PreviewURL() method - for example a dataobject which expects to belong to a page may return an empty value if it does not currently have a page relation set.

In this case the preview panel should display the "no preview available" message instead of spitting out a 404 - and ideally would be collapsed when loading this object (which is the same behaviour you would have for objects that are not CMSPreviewable.

Acceptance Criteria

• If a DataObject which implements CMSPreviewable has no preview URL in the current context, the preview panel displays a relevant message instead of an error page
• The preview panel is collapsed by default for the above mentioned situation
• When navigating to an edit form for a DataObject that does have a preview URL, the previously used mode (e.g. split, preview, or edit) is used (i.e. the above collapsing is not stored)

PRs

• FIX Allow empty PreviewURLs for CMSPreviewable objects silverstripe-cms#2737
• FIX Collapse the preview panel by default if no preview is available. #1305