fix(bundler): brace against undefined replace

lib/manager/bundler/extract.js

@@ -68,7 +68,7 @@ async function extractPackageFile(content, fileName) {
         lineNumber += 1;
         groupLine = lines[lineNumber];
         if (groupLine !== 'end') {
-          groupContent += groupLine.replace(/^ {2}/, '') + '\n';
+          groupContent += (groupLine || '').replace(/^ {2}/, '') + '\n';
         }
       }
       const groupRes = await extractPackageFile(groupContent);