Use empty string instead of NULL for mysqli SSL parameters by default

src/Driver/Mysqli/Driver.php

@@ -89,11 +89,11 @@ final class Driver extends AbstractMySQLDriver
             isset($params['ssl_cipher'])
         ) {
             $initializers[] = new Secure(
-                $params['ssl_key']    ?? null,
-                $params['ssl_cert']   ?? null,
-                $params['ssl_ca']     ?? null,
-                $params['ssl_capath'] ?? null,
-                $params['ssl_cipher'] ?? null
+                $params['ssl_key']    ?? '',
+                $params['ssl_cert']   ?? '',
+                $params['ssl_ca']     ?? '',
+                $params['ssl_capath'] ?? '',
+                $params['ssl_cipher'] ?? ''
             );
         }
 

src/Driver/Mysqli/Initializer/Secure.php

@@ -9,22 +9,22 @@ use mysqli;
 
 final class Secure implements Initializer
 {
-    /** @var string|null */
+    /** @var string */
     private $key;
 
-    /** @var string|null */
+    /** @var string */
     private $cert;
 
-    /** @var string|null */
+    /** @var string */
     private $ca;
 
-    /** @var string|null */
+    /** @var string */
     private $capath;
 
-    /** @var string|null */
+    /** @var string */
     private $cipher;
 
-    public function __construct(?string $key, ?string $cert, ?string $ca, ?string $capath, ?string $cipher)
+    public function __construct(string $key, string $cert, string $ca, string $capath, string $cipher)
     {
         $this->key    = $key;
         $this->cert   = $cert;