Change window_min_size from Size<Pixels> to Option<Size<Pixels>> (#13501)

Now we can set `window_min_size` to `None` instead of `Size::default()`.
I think this makes more sense.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -105,10 +105,10 @@ pub fn build_window_options(display_uuid: Option<Uuid>, cx: &mut AppContext) ->
         display_id: display.map(|display| display.id()),
         window_background: cx.theme().window_background_appearance(),
         app_id: Some(app_id.to_owned()),
-        window_min_size: gpui::Size {
+        window_min_size: Some(gpui::Size {
             width: px(360.0),
             height: px(240.0),
-        },
+        }),
     }
 }