Fix errors reported by phpstan in Database package.

src/Database/Schema/MysqlSchema.php

@@ -22,6 +22,12 @@ use Cake\Database\Schema\TableSchema;
  */
 class MysqlSchema extends BaseSchema
 {
+    /**
+     * The driver instance being used.
+     *
+     * @var \Cake\Database\Driver\Mysql
+     */
+    protected $_driver;
 
     /**
      * {@inheritDoc}

src/Database/Statement/SqliteStatement.php

@@ -48,9 +48,7 @@ class SqliteStatement extends StatementDecorator
      */
     public function rowCount()
     {
-        if ($this->_statement instanceof BufferedStatement
-            && preg_match('/^(?:DELETE|UPDATE|INSERT)/i', $this->_statement->queryString)
-        ) {
+        if (preg_match('/^(?:DELETE|UPDATE|INSERT)/i', $this->_statement->queryString)) {
             $changes = $this->_driver->prepare('SELECT CHANGES()');
             $changes->execute();
             $count = $changes->fetch()[0];

src/Database/StatementInterface.php

@@ -17,6 +17,8 @@ namespace Cake\Database;
 /**
  * Represents a database statement. Concrete implementations
  * can either use PDOStatement or a native driver
+ *
+ * @property string $queryString
  */
 interface StatementInterface
 {