fix: regression NPE creating new app.json

Author: quintesse

src/main/java/org/codejive/jpm/json/AppInfo.java

@@ -16,7 +16,7 @@
  * from/to files.
  */
 public class AppInfo {
-    private Map<String, Object> json;
+    private Map<String, Object> json = new TreeMap<>();
     public Map<String, String> dependencies = new TreeMap<>();
 
     /** The official name of the app.json file. */