bail on empty config

We will improve this in a future commit so we can still migrate `@apply`
for example in Tailwind CSS v4 -> v4 migrations.

Author: RobinMalfait

packages/@tailwindcss-upgrade/src/index.ts

@@ -155,7 +155,7 @@ async function run() {
     await Promise.all(
       stylesheets.map(async (sheet) => {
         try {
-          let config = configBySheet.get(sheet)!
+          let config = configBySheet.get(sheet)
           let jsConfigMigration = jsConfigMigrationBySheet.get(sheet)
 
           if (!config) {
@@ -168,6 +168,8 @@ async function run() {
             }
           }
 
+          if (!config) return
+
           await migrateStylesheet(sheet, {
             ...config,
             jsConfigMigration: jsConfigMigration ?? null,