Added Support in Query: Querying content with null

Added testcase to test null support in addParams() function
updated test package name to access protected members for better testing experience.

Author: shaileshmishra

pom.xml

@@ -6,7 +6,7 @@
     <modelVersion>4.0.0</modelVersion>
     <groupId>com.contentstack.sdk</groupId>
     <artifactId>java</artifactId>
-    <version>1.5.6</version>
+    <version>1.5.7-SNAPSHOT</version>
     <packaging>jar</packaging>
     <name>contentstack-java</name>
     <description>Java SDK for Contentstack Content Delivery API, Contentstack is a headless CMS with an API-first approach