use optionals, remove undefined enums

src/browser/main/tab/page/meta.rs

@@ -1,24 +1,23 @@
 use gtk::glib::GString;
 
 pub enum Mime {
-    Undefined,
     TextGemini,
     TextPlain,
 }
 
 pub struct Meta {
-    pub title: GString,
-    pub description: GString,
-    pub mime: Mime,
+    pub title: Option<GString>,
+    pub description: Option<GString>,
+    pub mime: Option<Mime>,
     pub progress_fraction: f32,
 }
 
 impl Meta {
     pub fn new() -> Self {
         Self {
-            title: GString::new(),
-            description: GString::new(),
-            mime: Mime::Undefined,
+            title: None,
+            description: None,
+            mime: None,
             progress_fraction: 0.0,
         }
     }