:bug: Fix NPE

kernel/sql/asset_content_query.go

···
 	if "" == query {
 		return nil, errors.New("statement is empty")
 	}
+	if nil == assetContentDB {
+		return nil, errors.New("database is nil")
+	}
 	return assetContentDB.Query(query, args...)
 }

kernel/sql/database.go

···
 		logging.LogErrorf("statement is empty")
 		return nil
 	}
+	if nil == db {
+		return nil
+	}
 	return db.QueryRow(query, args...)
 }
 
···
 	query = strings.TrimSpace(query)
 	if "" == query {
 		return nil, errors.New("statement is empty")
+	}
+	if nil == db {
+		return nil, errors.New("database is nil")
 	}
 	return db.Query(query, args...)
 }

kernel/sql/history.go

···
 }
 
 func SelectHistoriesRawStmt(stmt string) (ret []*History) {
+	if nil == historyDB {
+		return
+	}
 	rows, err := historyDB.Query(stmt)
 	if err != nil {
 		logging.LogWarnf("sql query [%s] failed: %s", stmt, err)
···
 	query = strings.TrimSpace(query)
 	if "" == query {
 		return nil, errors.New("statement is empty")
+	}
+	if nil == historyDB {
+		return nil, errors.New("database is nil")
 	}
 	return historyDB.Query(query, args...)
 }