Fix Pro dummy webpack config scss undefined error

Applied the same fix as spec/dummy to prevent webpack config error when scss
rule is not found in Shakapacker 9.3.0's generated config.

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

Fixes build-dummy-app-webpack-test-bundles failure:
  TypeError: Cannot read properties of undefined (reading 'push')
  at commonWebpackConfig.js:32

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

react_on_rails_pro/spec/dummy/config/webpack/commonWebpackConfig.js

@@ -29,7 +29,9 @@ const baseClientWebpackConfig = generateWebpackConfig();
 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);
+}
 
 if (isHMR) {
   baseClientWebpackConfig.plugins.push(