chore: add jsonInclude annotation and private constructor

src/main/java/io/github/doocs/im/model/group/AppDefinedDataItem.java

 package io.github.doocs.im.model.group;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 /**
  * @author bingo
  * @since 2021/10/31 20:57
  */
+@JsonInclude(JsonInclude.Include.NON_NULL)
 public class AppDefinedDataItem {
     @JsonProperty("Key")
     private String key;

src/main/java/io/github/doocs/im/model/group/AppMemberDefinedDataItem.java

 package io.github.doocs.im.model.group;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 /**
  * @author bingo
  * @since 2021/10/31 21:18
  */
+@JsonInclude(JsonInclude.Include.NON_NULL)
 public class AppMemberDefinedDataItem {
     @JsonProperty("Key")
     private String key;
@@ -47,7 +49,6 @@ public class AppMemberDefinedDataItem {
     }
 
 
-
     @Override
     public String toString() {
         return "AppDefinedDataItem{" +

src/main/java/io/github/doocs/im/model/group/GroupInfo.java

 package io.github.doocs.im.model.group;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 import java.util.List;
@@ -8,6 +9,7 @@ import java.util.List;
  * @author bingo
  * @since 2021/10/31 21:02
  */
+@JsonInclude(JsonInclude.Include.NON_NULL)
 public class GroupInfo {
     @JsonProperty("Owner_Account")
     private String ownerAccount;

src/main/java/io/github/doocs/im/model/response/GenericResult.java

@@ -51,7 +51,7 @@ public abstract class GenericResult {
 
     @Override
     public String toString() {
-        return "GenericResult{" +
+        return "Result{" +
                 "actionStatus='" + actionStatus + '\'' +
                 ", errorInfo='" + errorInfo + '\'' +
                 ", errorCode=" + errorCode +

src/main/java/io/github/doocs/im/util/VersionInfoUtil.java

@@ -17,6 +17,9 @@ public class VersionInfoUtil {
     private static final String VERSION_INFO_FILE = "version.properties";
     private static final String USER_AGENT_PREFIX = "qcloud-im-server-sdk-java";
 
+    private VersionInfoUtil() {
+    }
+
     public static String getDefaultUserAgent() {
         if (defaultUserAgent == null) {
             String osName = System.getProperty("os.name");