Fix: (org-ql--select) Don't search buffers without headings

Especially important when using org-ql-search to search "all" Org
buffers.

README.org

@@ -385,6 +385,7 @@ Expands into a call to ~org-ql-select~ with the same arguments.  For convenience
 *Fixed*
 +  Handle date ranges in date-based selectors.  (Thanks to [[https://github.com/codygman][Cody Goodman]], [[https://github.com/swflint][Samuel W. Flint]], and [[https://github.com/vikasrawal][Vikas Rawal]].)
 +  Don't overwrite bindings in =org-agenda-mode-map=.
++  Don't search buffers without headings, and show a message if the user attempts it.
 
 *Compatibility*
 +  Fixes for compatibility with Org 9.2.  (Thanks to [[https://github.com/ataias][Ataias Pereira Reis]] and [[https://github.com/dakra][Daniel Kraus]].)

org-ql.el

@@ -487,6 +487,14 @@ If NARROW is non-nil, buffer will not be widened."
               (goto-char (point-min))
               (when (org-before-first-heading-p)
                 (outline-next-heading))
+              (if (not (org-at-heading-p))
+                  (progn
+                    ;; No headings in buffer: return nil.
+                    (unless (string-prefix-p " " (buffer-name))
+                      ;; Not a special, hidden buffer: show message, because if a user accidentally
+                      ;; searches a buffer without headings, he might be confused.
+                      (message "org-ql: No headings in buffer: %s" (current-buffer)))
+                    nil)
                 ;; `cl-loop' makes this double-while much clearer than the expanded form.
                 (cond (preamble-re (cl-loop while (re-search-forward preamble-re nil t)
                                             do (outline-back-to-heading 'invisible-ok)
@@ -495,7 +503,7 @@ If NARROW is non-nil, buffer will not be widened."
                                             do (outline-next-heading)))
                       (t (cl-loop when (funcall predicate)
                                   collect (funcall action)
-                                while (outline-next-heading)))))))
+                                  while (outline-next-heading))))))))
       (--each orig-fns
         ;; Restore original function mappings.
         (fset (plist-get it :name) (plist-get it :fn))))))