Fix wrong value of when and unless with empty body (bug#74215)

Reported by Brennan Vincent.

* lisp/subr.el (when, unless): Return nil when the body is empty.
* test/lisp/subr-tests.el (subr-test-when): Add test cases.

lisp/subr.el

@@ -299,7 +299,7 @@ value of last one, or nil if there are none."
   (if body
       (list 'if cond (cons 'progn body))
     (macroexp-warn-and-return (format-message "`when' with empty body")
-                              cond '(empty-body when) t)))
+                              (list 'progn cond nil) '(empty-body when) t)))
 
 (defmacro unless (cond &rest body)
   "If COND yields nil, do BODY, else return nil.
@@ -309,7 +309,7 @@ value of last one, or nil if there are none."
   (if body
       (cons 'if (cons cond (cons nil body)))
     (macroexp-warn-and-return (format-message "`unless' with empty body")
-                              cond '(empty-body unless) t)))
+                              (list 'progn cond nil) '(empty-body unless) t)))
 
 (defsubst subr-primitive-p (object)
   "Return t if OBJECT is a built-in primitive written in C.

test/lisp/subr-tests.el

@@ -454,7 +454,11 @@
                    x)))
     (should (= x 2)))
   (should (equal (macroexpand-all '(when a b c d))
-                 '(if a (progn b c d)))))
+                 '(if a (progn b c d))))
+  (with-suppressed-warnings ((empty-body when unless))
+    (should (equal (when t) nil))
+    (should (equal (unless t) nil))
+    (should (equal (unless nil) nil))))
 
 (ert-deftest subr-test-xor ()
   "Test `xor'."