org-fontify-meta-lines-and-blocks-1: Fontify src blocks without lang

* lisp/org.el (org-fontify-meta-lines-and-blocks-1): Allow fontifying
src blocks with language specification missing.
* lisp/org-src.el (org-src-block-faces): Clarify in the docstring that
"" language name referes to src blocks with no language.  Remove
redundant :version tag.
* etc/ORG-NEWS (~org-src-block-faces~ now accepts empty string ~""~ as
language name): Announce the change.

Link: https://orgmode.org/list/99388ff3711696091f0312a5c3f0bcbd@kragelund.me

etc/ORG-NEWS

@@ -24,6 +24,21 @@ consider [[https://gitlab.com/jackkamm/ob-python-mode-mode][ob-python-mode-mode]
 has been ported to.
 
 ** New and changed options
+*** ~org-src-block-faces~ now accepts empty string ~""~ as language name
+
+It is now possible to customize face of source blocks without language specifier.
+
+: #+begin_src
+: Source block with no language
+: #+end_src
+
+For example, to set ~highlight~ face, use
+
+#+begin_src emacs-lisp
+(setq org-src-fontify-natively t)
+(add-to-list 'org-src-block-faces '("" highlight))
+#+end_src
+
 *** New ~org-cite-natbib-export-bibliography~ option defining fallback bibliography style
 
 ~natbib~ citation export processor now uses

lisp/org-src.el

@@ -233,7 +233,8 @@ Each element is a cell of the format
 
      (\"language\" FACE)
 
-Where FACE is either a defined face or an anonymous face.
+Where FACE is either a defined face or an anonymous face.  Empty
+language string refers to source blocks without specified language.
 
 For instance, the following would color the background of
 emacs-lisp source blocks and python source blocks in purple and
@@ -247,7 +248,6 @@ green, respectability.
                        (choice
                         (face :tag "Face")
                         (sexp :tag "Anonymous face"))))
-  :version "26.1"
   :package-version '(Org . "9.0"))
 
 (defcustom org-src-tab-acts-natively t

lisp/org.el

@@ -5415,9 +5415,9 @@ by a #."
 	    (org-remove-flyspell-overlays-in beg bol-after-beginline)
 	    (org-remove-flyspell-overlays-in nl-before-endline end-of-endline)
 	    (cond
-	     ((and lang (not (string= lang "")) org-src-fontify-natively)
+	     (org-src-fontify-natively
 	      (save-match-data
-                (org-src-font-lock-fontify-block lang block-start block-end))
+                (org-src-font-lock-fontify-block (or lang "") block-start block-end))
 	      (add-text-properties bol-after-beginline block-end '(src-block t)))
 	     (quoting
 	      (add-text-properties