encode empty arrays as nulls by default, leave old function as a backup

closes mitreid-connect#1011

Author: jricher

openid-connect-common/src/main/java/org/mitre/util/JsonUtils.java

@@ -35,6 +35,7 @@
 import com.google.common.collect.Sets;
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
+import com.google.gson.JsonNull;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonSyntaxException;
 import com.google.gson.reflect.TypeToken;
@@ -61,14 +62,30 @@ public class JsonUtils {
 private static Gson gson = new Gson();
 
 /**
- * Translate a set of strings to a JSON array
+ * Translate a set of strings to a JSON array, empty array returned as null
  * @param value
  * @return
  */
 public static JsonElement getAsArray(Set<String> value) {
-return gson.toJsonTree(value, new TypeToken<Set<String>>(){}.getType());
+return getAsArray(value, false);
 }
 
+
+/**
+ * Translate a set of strings to a JSON array, optionally preserving the empty array. Otherwise (default) empty array is returned as null.
+ * @param value
+ * @param preserveEmpty
+ * @return
+ */
+public static JsonElement getAsArray(Set<String> value, boolean preserveEmpty) {
+if (!preserveEmpty && value != null && value.isEmpty()) {
+// if we're not preserving empty arrays and the value is empty, return null
+return JsonNull.INSTANCE;
+} else {
+return gson.toJsonTree(value, new TypeToken<Set<String>>(){}.getType());
+}
+}
+
 /**
  * Gets the value of the given member (expressed as integer seconds since epoch) as a Date
  */