fix: crash on any backticks code block without lang specified

Author: RomanHotsiy

src/services/__tests__/prism.test.ts

@@ -16,4 +16,8 @@ describe('prism.js helpers', () => {
   test('highlight raw text should just return text', () => {
     expect(highlight('Hello world', 'clike')).toBe('Hello world');
   });
+
+  test('highlight should not throw with lang undefined', () => {
+    expect(highlight('Hello world', undefined)).toBe('Hello world');
+  });
 });

src/utils/highlight.ts

@@ -42,7 +42,7 @@ export function mapLang(lang: string): string {
  * @param lang highlight language
  * @return highlighted souce code as **html string**
  */
-export function highlight(source: string, lang: string): string {
+export function highlight(source: string, lang: string = DEFAULT_LANG): string {
   lang = lang.toLowerCase();
   let grammar = Prism.languages[lang];
   if (!grammar) {