Fix PHP 8 deprecations.

Author: peter17

src/ConnectionTrait.php

@@ -86,7 +86,7 @@ public function setGoneAwayDetector(GoneAwayDetector $goneAwayDetector): void
      *
      * @return R
      */
-    private function doWithRetry(callable $callable, string $sql = null)
+    private function doWithRetry(callable $callable, ?string $sql = null)
     {
         try {
             attempt:
@@ -187,7 +187,7 @@ public function beginTransaction(): void
         $this->currentlyOpeningFirstLevelTransaction = false;
     }
 
-    public function canTryAgain(\Throwable $throwable, string $sql = null): bool
+    public function canTryAgain(\Throwable $throwable, ?string $sql = null): bool
     {
         if ($this->hasBeenClosedWithAnOpenTransaction && ! $this->currentlyOpeningFirstLevelTransaction) {
             return false;

src/Detector/GoneAwayDetector.php

@@ -4,5 +4,5 @@
 
 interface GoneAwayDetector
 {
-    public function isGoneAwayException(\Throwable $exception, string $sql = null): bool;
+    public function isGoneAwayException(\Throwable $exception, ?string $sql = null): bool;
 }

src/Detector/MySQLGoneAwayDetector.php

@@ -17,7 +17,7 @@ class MySQLGoneAwayDetector implements GoneAwayDetector
         'Error while sending QUERY packet',
     ];
 
-    public function isGoneAwayException(\Throwable $exception, string $sql = null): bool
+    public function isGoneAwayException(\Throwable $exception, ?string $sql = null): bool
     {
         if ($this->isSavepoint($sql)) {
             return false;