getInsertId() is be able to return negative ID's [Closes #336]

src/Dibi/Connection.php

@@ -315,7 +315,7 @@ class Connection implements IConnection
 			$this->connect();
 		}
 		$id = $this->driver->getInsertId($sequence);
-		if ($id < 1) {
+		if ($id === null) {
 			throw new Exception('Cannot retrieve last generated ID.');
 		}
 		return $id;

src/Dibi/Drivers/MySqliDriver.php

@@ -199,7 +199,7 @@ class MySqliDriver implements Dibi\Driver
 	 */
 	public function getInsertId(?string $sequence): ?int
 	{
-		return $this->connection->insert_id;
+		return $this->connection->insert_id ?: null;
 	}
 
 

src/Dibi/Drivers/SqliteDriver.php

@@ -139,7 +139,7 @@ class SqliteDriver implements Dibi\Driver
 	 */
 	public function getInsertId(?string $sequence): ?int
 	{
-		return $this->connection->lastInsertRowID();
+		return $this->connection->lastInsertRowID() ?: null;
 	}