allow nullable _subtitle value, fix getValue data type returned

src/Entity/Browser/Container/Tab/Page/Title.php

@@ -66,12 +66,12 @@ class Title
         );
     }
 
-    public function getValue(): string
+    public function getValue(): ?string
     {
         return $this->gtk->get_text();
     }
 
-    public function getSubtitle(): string
+    public function getSubtitle(): ?string
     {
         return $this->gtk->get_subtitle();
     }

src/Gtk/Browser/Container/Tab/Page/Title/Label.php

@@ -6,16 +6,16 @@ namespace Yggverse\Yoda\Gtk\Browser\Container\Tab\Page\Title;
 
 class Label extends \GtkLabel
 {
-    private string $_subtitle = '';
+    private ?string $_subtitle = null;
 
     public function set_subtitle(
-        string $value
+        ?string $value = null
     ): void
     {
         $this->_subtitle = $value;
     }
 
-    public function get_subtitle(): string
+    public function get_subtitle(): ?string
     {
         return $this->_subtitle;
     }