Fix helm-occur transformer issue #2526

Avoid empty or nil candidates while not breaking nearest position.

helm-occur.el

@@ -252,17 +252,22 @@ engine beeing completely different and also much faster."
 (defun helm-occur-transformer (candidates source)
   "Return CANDIDATES prefixed with line number."
   (cl-loop with buf = (helm-get-attr 'buffer-name source)
-           for c in candidates collect
-           (when (string-match helm-occur--search-buffer-regexp c)
-             (let ((linum (match-string 1 c))
-                   (disp (match-string 2 c)))
-               (cons (format "%s:%s"
-                             (propertize
-                              linum 'face 'helm-grep-lineno
-                              'help-echo (buffer-file-name
-                                          (get-buffer buf)))
-                             disp)
-                     (string-to-number linum))))))
+           for c in candidates
+           for disp-linum = (when (string-match helm-occur--search-buffer-regexp c)
+                              (let ((linum (match-string 1 c))
+                                    (disp (match-string 2 c)))
+                                (list
+                                 linum
+                                 (format "%s:%s"
+                                         (propertize
+                                          linum 'face 'helm-grep-lineno
+                                          'help-echo (buffer-file-name
+                                                      (get-buffer buf)))
+                                         disp))))
+           for linum = (car disp-linum)
+           for disp = (cadr disp-linum)
+           when (and disp (not (string= disp "")))
+           collect (cons disp (string-to-number linum))))
 
 (defclass helm-moccur-class (helm-source-in-buffer)
   ((buffer-name :initarg :buffer-name