change(create-next-app): Get-pkg-manager logic change. (#50372)

Simplified Get-pkg-manager logic.

---------

Co-authored-by: Steven <steven@ceriously.com>

packages/create-next-app/helpers/get-pkg-manager.ts

@@ -1,17 +1,15 @@
 export type PackageManager = 'npm' | 'pnpm' | 'yarn'
 
 export function getPkgManager(): PackageManager {
-  const userAgent = process.env.npm_config_user_agent
+  const userAgent = process.env.npm_config_user_agent || ''
 
-  if (userAgent) {
-    if (userAgent.startsWith('yarn')) {
-      return 'yarn'
-    } else if (userAgent.startsWith('pnpm')) {
-      return 'pnpm'
-    } else {
-      return 'npm'
-    }
-  } else {
-    return 'npm'
+  if (userAgent.startsWith('yarn')) {
+    return 'yarn'
   }
+
+  if (userAgent.startsWith('pnpm')) {
+    return 'pnpm'
+  }
+
+  return 'npm'
 }