SQOOP-3435 Avoid NullPointerException due to different JSONObject lib…

…rary in classpath

Sqoop is expecting the classpath to have org.json [1] as the dependency
but if one uses HADOOP_CLASSPATH to initialize sqoop executor
with com.tdunning open json [2] as the resolution of org.json.JSONObject.
it failed with NullPointerException

1. https://mvnrepository.com/artifact/org.json/json/20090211
2. https://github.com/tdunning/open-json/blob/rc1.8/src/main/java/org/json/JSONObject.java#L141-L155

src/java/org/apache/sqoop/util/SqoopJsonUtil.java

@@ -18,6 +18,8 @@
 
 package org.apache.sqoop.util;
 
+import static java.util.Collections.emptyMap;
+
 import java.io.IOException;
 import java.util.Collections;
 import java.util.HashMap;
@@ -40,7 +42,8 @@ private SqoopJsonUtil() {
   }
 
   public static String getJsonStringforMap(Map<String, String> map) {
-    JSONObject pathPartMap = new JSONObject(map);
+    Map<String, String> mapToUse = (map == null) ? emptyMap() : map;
+    JSONObject pathPartMap = new JSONObject(mapToUse);
     return pathPartMap.toString();
   }
 
@@ -49,7 +52,7 @@ public static Map<String, String> getMapforJsonString(String mapJsonStr) {
     final Map<String, String> result;
     try {
       if (isEmptyJSON(mapJsonStr)) {
-        result = Collections.emptyMap();
+        result = emptyMap();
       } else {
         ObjectMapper mapper = new ObjectMapper();
         result = mapper.readValue(mapJsonStr,