Fix ein:worksheet-to-json

When there is no metadata, do not include it in JSON data to avoid
converting nil to JSON (which yields null).

lisp/ein-worksheet.el

@@ -228,7 +228,7 @@ current buffer."
                              c (ein:funcall-packed discard-output-p c)))
                           (ein:worksheet-get-cells ws)))))
     `((cells . ,(apply #'vector cells))
-      (metadata . ,(oref ws :metadata)))))
+      ,@(ein:aand (oref ws :metadata) `((metadata . ,it))))))
 
 (defmethod ein:worksheet-save-cells ((ws ein:worksheet))
   "Save cells in worksheet buffer in cache before killing the buffer.

tests/test-ein-worksheet.el

@@ -23,6 +23,9 @@
                   (ein:worksheet-from-json ws-1
                                            (ein:json-read-from-string
                                             (json-encode json-0))))))
+    (let* ((found (assoc 'metadata json-0)))
+      (when found
+        (should (cdr found))))
     (should (equal json-0 json-1))))
 
 (ert-deftest ein:worksheet-to-json/empty ()