Handle missing package description when unpacking vc packages

* lisp/emacs-lisp/package-vc.el (package-vc--unpack): Create a dummy
descriptor if PKG-DESC is nil.

lisp/emacs-lisp/package-vc.el

@@ -600,6 +600,8 @@ PKG-SPEC is a package specification, a property list describing
 how to fetch and build the package.  See `package-vc--archive-spec-alist'
 for details.  The optional argument REV specifies a specific revision to
 checkout.  This overrides the `:branch' attribute in PKG-SPEC."
+  (unless pkg-desc
+    (package-desc-create :name (car pkg-spec) :kind 'vc))
   (pcase-let* (((map :lisp-dir) pkg-spec)
                (name (package-desc-name pkg-desc))
                (dirname (package-desc-full-name pkg-desc))