org-src-font-lock-fontify-block: Do not transfer non-existing properties

* lisp/org-src.el (org-src-font-lock-fontify-block): Never set
font-lock properties when `get-text-property' returns nil.  This
usually means that the property is not present in the position
property list.  The old behavior could, for example, set 'invisible
property to nil explicitly and prevent folding completely.

Fixes https://list.orgmode.org/CA+G3_PN6RvcRcvN2fM=vfK1K6UsT=nNPJ7SUucmA1Ypc=Emorg@mail.gmail.com/T/#u

lisp/org-src.el

@@ -643,9 +643,10 @@ as `org-src-fontify-natively' is non-nil."
 	      ;; preserve, e.g., composition.
 	      (dolist (prop (cons 'face font-lock-extra-managed-props))
 		(let ((new-prop (get-text-property pos prop)))
-		  (put-text-property
-		   (+ start (1- pos)) (1- (+ start next)) prop new-prop
-		   org-buffer)))
+                  (when new-prop
+		    (put-text-property
+		     (+ start (1- pos)) (1- (+ start next)) prop new-prop
+		     org-buffer))))
 	      (setq pos next)))
           (set-buffer-modified-p nil))
 	;; Add Org faces.