Add dedicated warning for empty stdin (#9256)

## Summary

I ran into this in practice:

![Screenshot 2024-11-19 at 4 20
37 PM](https://github.com/user-attachments/assets/d89aa9f1-828a-492e-af5a-3434e277580e)

crates/uv-requirements-txt/src/lib.rs

@@ -220,11 +220,15 @@ impl RequirementsTxt {
             error: err,
         })?;
         if data == Self::default() {
+            if requirements_txt == Path::new("-") {
+                warn_user!("No dependencies found in stdin");
+            } else {
                 warn_user!(
                     "Requirements file {} does not contain any dependencies",
                     requirements_txt.user_display()
                 );
             }
+        }
 
         Ok(data)
     }