[MongoDB] Fix NullPointerException when executing ddl operation in MongoDB

[e-mhui]

[MongoDB] Fix NullPointerException when executing ddl operation in MongoDB

Fix: #2106

When performing DDL operations in MongoDB, documentKey and fullDocument are null, so we need to check if it is a DML operation and retrieve documentKey and fullDocument from the record.

    if (isDMLOperation(op)) {
        documentKey =
                checkNotNull(
                        extractBsonDocument(
                                value, valueSchema, MongoDBEnvelope.DOCUMENT_KEY_FIELD));
        fullDocument =
                extractBsonDocument(value, valueSchema, MongoDBEnvelope.FULL_DOCUMENT_FIELD);
    }

[Jiabao-Sun]

Thanks @e-mhui for report this problem.
I think delaying the empty check can also solve this problem.