don't show top dashed border on wikibase-toolbar-wrapper for new statements

doesn't look nice otherwise when clicking "add" on an item with
no statements or to add a new one.

Change-Id: I488d30baac53a8c590b27eb242f08157096c07ca
(cherry picked from commit ad652806e89c2f413e18cdb7597ce5366a8ae93f)