ox: Fix exporting code blocks with missing language specifier

* lisp/ox-latex.el (org-latex-inline-src-block): Export code blocks
without language as verbatim.
* lisp/ox-man.el (org-man-inline-src-block):
(org-man-src-block): Do not try to call source-highlight executable
when code block language is missing.
* lisp/ox-odt.el (org-odt-do-format-code):
* lisp/ox-texinfo.el (org-texinfo-src-block): Do not throw error when
code block language is missing.

Reproducer (not available on the list):

  To reproduce my problem you can write create an org-mode buffer (C-x b
  1.org M-x org-mode) with the following content:

  #+begin_src lua :tangle yes  :noweb yes
    aaa
    <<second>>
  #+end_src

  #+name: second
  #+begin_src
   bbb
  #+end_src

  Then enable exporting to texinfo and export the buffer:
    M-: (require 'ox-texinfo)
    C-c C-e i t

  The folllowing backtrace appears (providing you already enabled
  debugging on errors):

  Debugger entered--Lisp error: (wrong-type-argument stringp nil)
    apply(debug error (wrong-type-argument stringp nil))
    edebug(error (wrong-type-argument stringp nil))
    signal(wrong-type-argument (stringp nil))
    edebug-signal(wrong-type-argument (stringp nil))
    string-match("lisp" nil nil t)
    string-match-p("lisp" nil)
    (edebug-after (edebug-before 1) 5 (string-match-p "lisp" (edebug-after (edebug-before 2) 4
  (org-element-property :language (edebug-after 0 3 src-block)))))

Reported-by: pva-outdoor@yandex.ru
Link: https://list.orgmode.org/orgmode/87jzn7oz1u.fsf@yandex.ru/

lisp/ox-latex.el

@@ -2451,7 +2451,7 @@ contextual information."
   (let ((code (org-element-property :value inline-src-block))
         (lang (org-element-property :language inline-src-block)))
     (pcase (plist-get info :latex-src-block-backend)
-      (`verbatim (org-latex--text-markup code 'code info))
+      ((or `verbatim (guard (not lang))) (org-latex--text-markup code 'code info))
       (`minted (org-latex-inline-src-block--minted info code lang))
       (`engraved (org-latex-inline-src-block--engraved info code lang))
       (`listings (org-latex-inline-src-block--listings info code lang))

lisp/ox-man.el

@@ -510,8 +510,9 @@ contextual information."
                         (expand-file-name "reshilite" tmpdir)))
              (org-lang (org-element-property :language inline-src-block))
              (lst-lang
-	      (cadr (assq (intern org-lang)
-			  (plist-get info :man-source-highlight-langs))))
+              (and org-lang
+	           (cadr (assq (intern org-lang)
+			       (plist-get info :man-source-highlight-langs)))))
 
              (cmd (concat (expand-file-name "source-highlight")
                           " -s " lst-lang
@@ -757,8 +758,9 @@ contextual information."
 	   (code (org-element-property :value src-block))
 	   (org-lang (org-element-property :language src-block))
 	   (lst-lang
-	    (cadr (assq (intern org-lang)
-			(plist-get info :man-source-highlight-langs))))
+            (and org-lang
+	         (cadr (assq (intern org-lang)
+			     (plist-get info :man-source-highlight-langs)))))
 	   (cmd (concat "source-highlight"
 			" -s " lst-lang
 			" -f groff_man "

lisp/ox-odt.el

@@ -3111,7 +3111,7 @@ and prefix with \"OrgSrc\".  For example,
 (defun org-odt-do-format-code
     (code info &optional lang refs retain-labels num-start)
   (let* ((lang (or (assoc-default lang org-src-lang-modes) lang))
-	 (lang-mode (and lang (intern (format "%s-mode" lang))))
+	 (lang-mode (if lang (intern (format "%s-mode" lang)) #'ignore))
 	 (code-lines (org-split-string code "\n"))
 	 (code-length (length code-lines))
 	 (use-htmlfontify-p (and (functionp lang-mode)

lisp/ox-texinfo.el

@@ -1681,8 +1681,9 @@ as a communication channel."
   "Transcode a SRC-BLOCK element from Org to Texinfo.
 CONTENTS holds the contents of the item.  INFO is a plist holding
 contextual information."
-  (let* ((lisp (string-match-p "lisp"
-			       (org-element-property :language src-block)))
+  (let* ((lisp (string-match-p
+                "lisp"
+		(or (org-element-property :language src-block) "")))
 	 (code (org-texinfo--sanitize-content
 		(org-export-format-code-default src-block info)))
 	 (value (format