json: Work around bug for (scm->json '()).

* jupyter/json.scm <top level>: Override (@@ (json builder) json-build).

jupyter/json.scm

@@ -87,3 +87,13 @@ to its JSON representation (an alist)."
 
        (define-json-reader json->record ctor
          (field spec ...) ...)))))
+
+(set! (@@ (json builder) json-build)
+  ;; Work around a bug in Guile-JSON 3.1.0:
+  ;; <https://github.com/aconchillo/guile-json/issues/47>.
+  (let ((real-json-build (@@ (json builder) json-build)))
+
+    (lambda (scm port . rest)
+      (if (eq? scm '())                           ;match '() but not #nil
+          (display "{}" port)
+          (apply real-json-build scm port rest)))))