Fix webpack config error with undefined scss rule

The webpack config was failing with "Cannot read properties of undefined
(reading 'push')" because the SCSS rule might not exist or might not have
a 'use' property in Shakapacker 9.3.0's generated config.

Added proper guard check before attempting to push to the scss config's
use array.

Error was:
TypeError: Cannot read properties of undefined (reading 'push')
at commonWebpackConfig.js:27:59

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/dummy/config/webpack/commonWebpackConfig.js

@@ -24,7 +24,9 @@ const sassLoaderConfig = {
 const scssConfigIndex = baseClientWebpackConfig.module.rules.findIndex((config) =>
   '.scss'.match(config.test),
 );
-baseClientWebpackConfig.module.rules[scssConfigIndex]?.use.push(sassLoaderConfig);
+if (scssConfigIndex !== -1 && baseClientWebpackConfig.module.rules[scssConfigIndex]?.use) {
+  baseClientWebpackConfig.module.rules[scssConfigIndex].use.push(sassLoaderConfig);
+}
 
 // Configure CSS Modules to use default exports (Shakapacker 9.0 compatibility)
 // Shakapacker 9.0 defaults to namedExport: true, but we use default imports