json: if-json-null combinator, in the same vein as if-zero, if-empty, etc.

basis/json/json.factor

@@ -1,7 +1,15 @@
+USING: kernel vocabs.loader ;
 IN: json
-USE: vocabs.loader
 
 SINGLETON: json-null
 
+: if-json-null ( x if-null else -- )
+    [ dup json-null? ]
+    [ [ drop ] prepose ]
+    [ ] tri* if ; inline
+
+: when-json-null ( x if-null -- ) [ ] if-json-null ; inline
+: unless-json-null ( x else -- ) [ ] swap if-json-null ; inline
+
 "json.reader" require
 "json.writer" require