Add 'nnregistry-request-group' function for 'gnus-refer-article'

Add 'nnregistry-request-group' function for 'gnus-refer-article'

nnregistry doesn't handle any servers or groups; however we have to
define the dummy function so Gnus-registry works as refer article
method, as the `gnus-check-group-server' function requires it.
The latter function is called when referring to articles to check the
source group first before actually requesting any articles.
* lisp/gnus/nnregistry.el (nnregistry-request-group): New deffoo.
(Bug#22414)

(cherry picked from commit 628e3cb22d196c31fc2452c3eab4a62af4bf66b2)