Fix BaseMap::$instance BC issue (#85)

Author: mondrake

src/Map/BaseMap.php

@@ -13,8 +13,10 @@ abstract class BaseMap implements MapInterface
 {
     /**
      * Singleton instance.
+     *
+     * @var ?MapInterface
      */
-    protected static ?MapInterface $instance;
+    protected static $instance;
 
     /**
      * Mapping between file extensions and MIME types.

src/Map/DefaultMap.php

@@ -9,7 +9,7 @@
  */
 class DefaultMap extends AbstractMap
 {
-    protected static ?MapInterface $instance;
+    protected static $instance;
 
     public function getFileName(): string
     {

src/Map/EmptyMap.php

@@ -10,5 +10,5 @@
  */
 class EmptyMap extends AbstractMap
 {
-    protected static ?MapInterface $instance;
+    protected static $instance;
 }

tests/fixtures/MiniMap.php.test

@@ -9,7 +9,7 @@ namespace FileEye\MimeMap\Map;
  */
 class MiniMap extends AbstractMap
 {
-    protected static ?MapInterface $instance;
+    protected static $instance;
 
     public function getFileName(): string
     {