Remove nullable properties and assertions, use isset/unset and langua…

…ge constraints (#171)

src/Migration/LazyLoadingMigrations.php

@@ -13,11 +13,11 @@
 final class LazyLoadingMigrations implements Migrations
 {
     /**
-     * @var \Closure(): void|null
+     * @var \Closure(): void
      */
-    private ?\Closure $loader;
+    private \Closure $loader;
 
-    private ?Migrations $migrations = null;
+    private Migrations $migrations;
 
     /**
      * @param \Closure(): Migrations $loader
@@ -26,40 +26,34 @@ public function __construct(\Closure $loader)
     {
         $this->loader = function () use ($loader): void {
             $this->migrations = $loader->bindTo(null)();
-            $this->loader = null;
+            unset($this->loader);
         };
     }
 
     public function getAll(): VectorInterface
     {
-        if ($this->loader !== null) {
+        if (isset($this->loader)) {
             ($this->loader)();
         }
 
-        \assert($this->migrations instanceof Migrations);
-
         return $this->migrations->getAll();
     }
 
     public function get(Version $version): Migration
     {
-        if ($this->loader !== null) {
+        if (isset($this->loader)) {
             ($this->loader)();
         }
 
-        \assert($this->migrations instanceof Migrations);
-
         return $this->migrations->get($version);
     }
 
     public function changeState(Version $version, State $state): Migration
     {
-        if ($this->loader !== null) {
+        if (isset($this->loader)) {
             ($this->loader)();
         }
 
-        \assert($this->migrations instanceof Migrations);
-
         return $this->migrations->changeState($version, $state);
     }
 }