Update CognitoSecretHash.java

While creating CognitoUserPool instance using the awsConfiguration.json file with following description.
"CognitoUserPool": {
    "Default": {
      "PoolId": "xxxxxxxxxx",
      "AppClientId": "xxxxxxxx",
      "AppClientSecret": "",
      "Region": "xxxx"
    }
  }

It will return appClientSecret as "" which is not checked in this file and cause crash in android application.

Author: NischalManandhar

aws-android-sdk-cognitoidentityprovider/src/main/java/com/amazonaws/mobileconnectors/cognitoidentityprovider/util/CognitoSecretHash.java

@@ -49,8 +49,9 @@ public static String getSecretHash(String userId, String clientId, String client
             throw new CognitoParameterInvalidException("client ID cannot be null");
         }
 
-        // Return null as secret hash if clientSecret is null.
-        if (clientSecret == null) {
+        // Return null as secret hash if clientSecret is null or "".
+        // We receive clientSecret as "" if we create cognitoUserPool from awsConfiguration file with "AppClientSecret": "
+        if (TextUtils.isEmpty(clientSecret)) {
             return null;
         }