input: Fix text not in vertical center when masked text is empty. (#1753)

Closes #1724

## Screenshot

### Before                      
<img width="824" height="656" alt="CleanShot20251206132455"
src="https://github.com/user-attachments/assets/dc917dc2-1c48-4a7a-bc24-29f09f656d95"
/>

### After 
<img width="826" height="654" alt="CleanShot20251206132600"
src="https://github.com/user-attachments/assets/3a62e7fd-1f85-4165-84bf-7a9b25f5ef3d"
/>

---------

Co-authored-by: Jason Lee <huacnlee@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

crates/story/src/input_story.rs

@@ -65,6 +65,7 @@ impl InputStory {
         let mask_input = cx.new(|cx| {
             InputState::new(window, cx)
                 .masked(true)
+                .placeholder("Enter your password...")
                 .default_value("this-is-password-中文🚀🎉")
         });
 

crates/ui/src/input/element.rs

@@ -1179,7 +1179,8 @@ impl Element for TextElement {
         let invisible_top_padding = prepaint.last_layout.visible_top;
 
         let mut mask_offset_y = px(0.);
-        if self.state.read(cx).masked {
+        let state = self.state.read(cx);
+        if state.masked && state.text.len() > 0 {
             // Move down offset for vertical centering the *****
             if cfg!(target_os = "macos") {
                 mask_offset_y = px(3.);