Fixed non-existing keys not being set with default values in imported config

module/Installer/test/ConfigProviderTest.php

@@ -0,0 +1,29 @@
+<?php
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Installer;
+
+use PHPUnit\Framework\TestCase;
+use Shlinkio\Shlink\Installer\ConfigProvider;
+
+class ConfigProviderTest extends TestCase
+{
+    /**
+     * @var ConfigProvider
+     */
+    protected $configProvider;
+
+    public function setUp()
+    {
+        $this->configProvider = new ConfigProvider();
+    }
+
+    /**
+     * @test
+     */
+    public function configIsReturned()
+    {
+        $config = $this->configProvider->__invoke();
+        $this->assertEmpty($config);
+    }
+}