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.rs

@@ -569,7 +569,7 @@ pub struct WindowOptions {
     pub app_id: Option<String>,
 
     /// Window minimum size
-    pub window_min_size: Size<Pixels>,
+    pub window_min_size: Option<Size<Pixels>>,
 }
 
 /// The variables that can be configured when creating a new window
@@ -599,7 +599,7 @@ pub(crate) struct WindowParams {
     pub window_background: WindowBackgroundAppearance,
 
     #[cfg_attr(target_os = "linux", allow(dead_code))]
-    pub window_min_size: Size<Pixels>,
+    pub window_min_size: Option<Size<Pixels>>,
 }
 
 /// Represents the status of how a window should be opened.
@@ -648,7 +648,7 @@ impl Default for WindowOptions {
             display_id: None,
             window_background: WindowBackgroundAppearance::default(),
             app_id: None,
-            window_min_size: Size::default(),
+            window_min_size: None,
         }
     }
 }