fix: allow TS filename to be undefined

fixes #488

src/transformers/typescript.ts

@@ -203,13 +203,11 @@ let warned_mixed = false;
 
 const transformer: Transformer<Options.Typescript> = async ({
   content,
-  filename,
+  filename = 'input.svelte',
   options = {},
 }) => {
   const basePath = process.cwd();
 
-  if (filename == null) return { code: content };
-
   filename = isAbsolute(filename) ? filename : resolve(basePath, filename);
 
   const compilerOptions = getCompilerOptions({ filename, options, basePath });