fix: check if filename exist

Author: micantoine

src/index.ts

@@ -35,7 +35,7 @@ let pluginOptions: PluginOptions;
  * @returns the preprocessor markup
  */
 const markup: MarkupPreprocessor = async ({ content, filename }) => {
-  const isIncluded = isFileIncluded(pluginOptions.includePaths, filename);
+  const isIncluded = filename ? isFileIncluded(pluginOptions.includePaths, filename) : false;
 
   if (!isIncluded || (!pluginOptions.parseStyleTag && !pluginOptions.parseExternalStylesheet)) {
     return { code: content };