Gnus: Fix edge cases with no data being returned and Courier eccentricity.

Gnus: Fix edge cases with no data being returned and Courier eccentricity.

* lisp/gnus/nnimap.el (nnimap-retrieve-headers-progress): Handle edge
  case where `headers' is nil.  This can occur if the IMAP server does
  not have permissions to read messages from a folder, but can write new
  messages to the folder.
  (nnimap-request-article-part): Do not insert `data' if it is nil.

* lisp/net/imap.el (imap-parse-fetch): Courier can insert spurious blank
  characters which will confuse `read', so skip past them.