simplify driver initialization using null coalescing assignment (#56210)

Author: Ashot1995

src/Illuminate/Support/Manager.php

@@ -75,11 +75,7 @@ public function driver($driver = null)
         // If the given driver has not been created before, we will create the instances
         // here and cache it so we can return it next time very quickly. If there is
         // already a driver created by this name, we'll just return that instance.
-        if (! isset($this->drivers[$driver])) {
-            $this->drivers[$driver] = $this->createDriver($driver);
-        }
-
-        return $this->drivers[$driver];
+        return $this->drivers[$driver] ??= $this->createDriver($driver);
     }
 
     /**