make the DesignSystem fully nullable

The `DesignSystem` was already nullable (or well, optional) but that
wasn't well reflected in the types.

This would otherwise break this test:
- `tailwindcss/integrations/upgrade/index.test.ts`
  - `migrate utility files imported by multiple roots`

Author: RobinMalfait

packages/@tailwindcss-upgrade/src/codemods/css/migrate-at-apply.ts

@@ -8,7 +8,7 @@ export function migrateAtApply({
   designSystem,
   userConfig,
 }: {
-  designSystem: DesignSystem
+  designSystem: DesignSystem | null
   userConfig: Config | null
 }): Plugin {
   function migrate(atRule: AtRule) {
@@ -35,6 +35,8 @@ export function migrateAtApply({
     })
 
     return async () => {
+      if (!designSystem) return
+
       // If we have a valid designSystem and config setup, we can run all
       // candidate migrations on each utility
       params = await Promise.all(

packages/@tailwindcss-upgrade/src/codemods/css/migrate-media-screen.ts

@@ -9,7 +9,7 @@ export function migrateMediaScreen({
   designSystem,
   userConfig,
 }: {
-  designSystem?: DesignSystem
+  designSystem?: DesignSystem | null
   userConfig?: Config | null
 } = {}): Plugin {
   function migrate(root: Root) {

packages/@tailwindcss-upgrade/src/codemods/css/migrate-preflight.ts

@@ -35,13 +35,14 @@ export function migratePreflight({
   designSystem,
   userConfig,
 }: {
-  designSystem: DesignSystem
+  designSystem: DesignSystem | null
   userConfig?: Config | null
 }): Plugin {
   // @ts-expect-error
   let defaultBorderColor = userConfig?.theme?.borderColor?.DEFAULT
 
   function canResolveThemeValue(path: string) {
+    if (!designSystem) return false
     let variable = `--${keyPathToCssProperty(toKeyPath(path))}` as const
     return Boolean(designSystem.theme.get([variable]))
   }

packages/@tailwindcss-upgrade/src/codemods/css/migrate-theme-to-var.ts

@@ -5,7 +5,7 @@ import { Convert, createConverter } from '../template/migrate-theme-to-var'
 export function migrateThemeToVar({
   designSystem,
 }: {
-  designSystem?: DesignSystem
+  designSystem?: DesignSystem | null
 } = {}): Plugin {
   return {
     postcssPlugin: '@tailwindcss/upgrade/migrate-theme-to-var',

packages/@tailwindcss-upgrade/src/codemods/css/migrate.ts

@@ -16,7 +16,7 @@ import { migrateVariantsDirective } from './migrate-variants-directive'
 
 export interface MigrateOptions {
   newPrefix: string | null
-  designSystem: DesignSystem
+  designSystem: DesignSystem | null
   userConfig: Config | null
   configFilePath: string | null
   jsConfigMigration: JSConfigMigration | null

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

@@ -123,7 +123,7 @@ async function run() {
     >()
     for (let sheet of stylesheets) {
       if (!sheet.isTailwindRoot) continue
-      if (!sheet.linkedConfigPath) continue
+      if (!version.isMajor(3) && !sheet.linkedConfigPath) continue
 
       let config = await prepareConfig(sheet.linkedConfigPath, { base })
       configBySheet.set(sheet, config)
@@ -168,20 +168,11 @@ async function run() {
             }
           }
 
-          let designSystem = config?.designSystem ?? (await sheet.designSystem())
-          if (!designSystem) {
-            return
-          }
-
-          let newPrefix = config?.newPrefix ?? null
-          let userConfig = config?.userConfig ?? null
-          let configFilePath = config?.configFilePath ?? null
-
           await migrateStylesheet(sheet, {
-            newPrefix,
-            designSystem,
-            userConfig,
-            configFilePath,
+            newPrefix: config?.newPrefix ?? null,
+            designSystem: config?.designSystem ?? (await sheet.designSystem()),
+            userConfig: config?.userConfig ?? null,
+            configFilePath: config?.configFilePath ?? null,
             jsConfigMigration,
           })
         } catch (e: any) {