types(compiler-sfc): add explicit return type to genModelProps (#13441)

packages/compiler-sfc/src/script/defineModel.ts

@@ -114,7 +114,7 @@ export function processDefineModel(
   return true
 }
 
-export function genModelProps(ctx: ScriptCompileContext) {
+export function genModelProps(ctx: ScriptCompileContext): string | undefined {
   if (!ctx.hasDefineModelCall) return
 
   const isProd = !!ctx.options.isProd