Avoid errors when a restricted-sexp widget is empty

* lisp/wid-edit.el (restricted-sexp): Don't try to read
an empty string when converting the current value to the
external format.  (Bug#63838)

* test/lisp/wid-edit-tests.el (widget-test-restricted-sexp-empty-val):
New test.

lisp/wid-edit.el

@@ -3681,7 +3681,9 @@ match-alternatives: %S"
                           :warning)
                          ;; Make sure we will `read' a string.
                          (setq value (prin1-to-string value)))
-                       (read value)))
+                       (if (string-empty-p value)
+                           value
+                       (read value))))
 
 (defun widget-restricted-sexp-match (widget value)
   (let ((alternatives (widget-get widget :match-alternatives))

test/lisp/wid-edit-tests.el

@@ -380,4 +380,15 @@ return nil, even with a non-nil bubblep argument."
                                     :value (("1" . 1) ("2" . 2))))))
       (should (equal '(("1" . 1) ("2" . 2)) (widget-default-get w))))))
 
+(ert-deftest widget-test-restricted-sexp-empty-val ()
+  "Test that we handle an empty restricted-sexp widget just fine."
+  (with-temp-buffer
+    (let ((w (widget-create '(restricted-sexp
+                              :value 3
+                              :match-alternatives (integerp)))))
+      (widget-setup)
+      (widget-backward 1)
+      (delete-char 1)
+      (should (string= (widget-value w) "")))))
+
 ;;; wid-edit-tests.el ends here