Added null checks to GCM service.

components/device-mgt/org.wso2.carbon.device.mgt.mobile.impl/src/main/java/org/wso2/carbon/device/mgt/mobile/impl/android/gcm/GCMUtil.java

@@ -123,7 +123,7 @@ public class GCMUtil {
         //Set device reg-ids
         JsonArray regIds = new JsonArray();
         for (String regId : registrationIds) {
-            if (regId == null && regId.isEmpty()) {
+            if (regId == null || regId.isEmpty()) {
                 continue;
             }
             regIds.add(new JsonPrimitive(regId));