fix(vite-plugin): sass variables for files with only @use statements (#13371)

* sass variables for files with only @use statements

scss-transform will fail currently for file like this:
```sass
@use 'A'
@use 'B' -- no new line here
```
gives
```
[vite] Internal server error: @use rules must be written before any other rules.
```

* Update scss-transform.js

Co-authored-by: Razvan Stoenescu <razvan.stoenescu@gmail.com>

Author: disfated

vite-plugin/src/scss-transform.js

@@ -23,7 +23,13 @@ export function createScssTransform (fileExtension, sassVariables) {
       return prefix + content
     }
 
-    const newLineIndex = content.indexOf('\n', useIndex) + 1
-    return content.substring(0, newLineIndex) + prefix + content.substring(newLineIndex)
+    const newLineIndex = content.indexOf('\n', useIndex)
+    
+    if (newLineIndex !== -1) {
+      const index = newLineIndex + 1
+      return content.substring(0, index) + prefix + content.substring(index)
+    }
+
+    return content + '\n' + prefix
   }
 }