org.el: Fix regression in handling of empty #+TAGS

* lisp/org.el (org-set-regexps-and-options): Allow an empty #+TAGS
value to override org-tag-alist, as it did before v9.4.
* testing/lisp/test-org.el (test-org/set-regexps-and-options): Add
test.

As of b4e91b7e9 (New function: org-collect-keywords, 2020-04-26), a
"#+TAGS" header can no longer be used to override a value of
org-tag-alist.  This breaks the workflow where a set of tags for most
files is defined via org-file-tags and then, in a particular file, an
empty #+TAGS header is used to ignore org-file-tags and trigger
collecting tags from the buffer instead.  Rework the handling to
restore this behavior.

Reported-by: Allen Li <darkfeline@felesatra.moe>
Ref: https://orgmode.org/list/80y2laly9v.fsf@felesatra.moe

lisp/org.el

@@ -4279,10 +4279,10 @@ related expressions."
       (setq org-current-tag-alist
 	    (org--tag-add-to-alist
 	     org-tag-persistent-alist
-	     (let ((tags (mapconcat #'identity
-				    (cdr (assoc "TAGS" alist))
-				    "\n")))
-	       (if (org-string-nw-p tags) (org-tag-string-to-alist tags)
+	     (let ((tags (cdr (assoc "TAGS" alist))))
+	       (if tags
+		   (org-tag-string-to-alist
+		    (mapconcat #'identity tags "\n"))
 		 org-tag-alist))))
       (setq org-tag-groups-alist
 	    (org-tag-alist-to-groups org-current-tag-alist))

testing/lisp/test-org.el

@@ -2457,6 +2457,11 @@ SCHEDULED: <2014-03-04 tue.>"
 	    (org-test-with-temp-text "#+TAGS: [ A : B C ]"
 	      (org-mode-restart)
 	      org-tag-groups-alist))))
+  (should-not
+   (let ((org-tag-alist '(("A"))))
+     (org-test-with-temp-text "#+TAGS:"
+       (org-mode-restart)
+       org-current-tag-alist)))
   ;; FILETAGS keyword.
   (should
    (equal '("A" "B" "C")