Working on improving JSON null handling

Relates to #9129

grails-plugin-converters/src/test/groovy/grails/converters/ParsingNullJsonValuesSpec.groovy

@@ -0,0 +1,12 @@
+package grails.converters
+
+import spock.lang.Specification
+
+class ParsingNullJsonValuesSpec extends Specification {
+
+    def "test parsing null value"() {
+        expect:
+        JSON.parse("{'myList':null}").myList == null
+        JSON.parse("{'myList':null}").get('myList') == null
+    }
+}

grails-web-common/src/main/groovy/org/codehaus/groovy/grails/web/json/JSONObject.java

@@ -331,7 +331,7 @@ public Object get(String key) throws JSONException {
             throw new JSONException("JSONObject[" + quote(key) +
                     "] not found.");
         }
-        return o;
+        return o instanceof Null ? null : o;
     }
 
 
@@ -1246,7 +1246,8 @@ public boolean containsValue(Object o) {
     }
 
     public Object get(Object o) {
-        return myHashMap.get(o);
+        Object value =  myHashMap.get(o);
+        return value instanceof Null ? null : value;
     }
 
     public Object put(Object o, Object o1) {