Fix nullable parameters for PHP 8.4

src/CryptoCurrencyChart/API/Client.php

@@ -90,7 +90,7 @@ class Client {
 		return $response['baseCurrencies'];
 	}
 
-	public function viewCoin(int $coinId, \DateTime $date = null, string $baseCurrency = 'USD'): Coin {
+	public function viewCoin(int $coinId, ?\DateTime $date = null, string $baseCurrency = 'USD'): Coin {
 		$response = $this->apiCall('viewCoin', [$coinId, $date, $baseCurrency]);
 		if (!isset($response['coin']) || !\is_array($response['coin'])) {
 			throw new ServerException('Invalid response for viewCoin, missing or invalid `coin`.');