make migrations_paths optional

Author: goetas

DependencyInjection/Configuration.php

@@ -48,8 +48,7 @@ public function getConfigTreeBuilder() : TreeBuilder
             ->children()
                 ->arrayNode('migrations_paths')
                     ->info('A list of namespace/path pairs where to look for migrations.')
-                    ->isRequired()
-                    ->requiresAtLeastOneElement()
+                    ->defaultValue([])
                     ->useAttributeAsKey('namespace')
                     ->prototype('scalar')->end()
                 ->end()

Tests/DependencyInjection/DoctrineMigrationsExtensionTest.php

@@ -87,20 +87,19 @@ public function testFullConfig() : void
         $this->assertConfigs($config);
     }
 
-    public function testNoConfig() : void
+    public function testNoConfigsAreNeeded() : void
     {
-        $this->expectException(InvalidConfigurationException::class);
-        $this->expectExceptionMessage('The child node "migrations_paths" at path "doctrine_migrations" must be configured.');
-
         $container = $this->getContainer([]);
 
         $conn = $this->createMock(Connection::class);
         $container->set('doctrine.dbal.default_connection', $conn);
         $container->compile();
 
-        $container->get('doctrine.migrations.configuration');
-    }
+        $config = $container->get('doctrine.migrations.configuration');
 
+        self::assertInstanceOf(Configuration::class, $config);
+        self::assertSame([], $config->getMigrationDirectories());
+    }
 
     private function assertConfigs(?object $config) : void
     {