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/gpui/src/platform/mac/window.rs

@@ -647,10 +647,12 @@ impl MacWindow {
 
             native_window.setMovable_(is_movable as BOOL);
 
-            native_window.setContentMinSize_(NSSize {
-                width: window_min_size.width.to_f64(),
-                height: window_min_size.height.to_f64(),
-            });
+            if let Some(window_min_size) = window_min_size {
+                native_window.setContentMinSize_(NSSize {
+                    width: window_min_size.width.to_f64(),
+                    height: window_min_size.height.to_f64(),
+                });
+            }
 
             if titlebar.map_or(true, |titlebar| titlebar.appears_transparent) {
                 native_window.setTitlebarAppearsTransparent_(YES);