🪲 fix level dropdown in custom-adventure page (hedyorg#5660)

fixing:

https://github.com/hedyorg/hedy/assets/20051470/fb007c32-43fc-4fc6-a04f-c8e5e2be5214


**How to test?**
- deselect all levels of an adventure,
- an alert about not selecting any level should appear

static/js/appbundle.js

@@ -118475,7 +118475,10 @@ def note_with_error(value, err):
   function get_formatted_content(content2, levels, language2) {
     const parser19 = new DOMParser();
     const html = parser19.parseFromString(content2, "text/html");
-    const minLevel = Math.min(...levels.map((el3) => Number(el3)));
+    let minLevel = 1;
+    if (levels.length) {
+      minLevel = Math.min(...levels.map((el3) => Number(el3)));
+    }
     let snippets = [];
     let snippetsFormatted = [];
     let keywords2 = [];

static/js/teachers.ts

@@ -218,7 +218,10 @@ export function remove_student(class_id: string, student_id: string, prompt: str
 function get_formatted_content(content: string, levels: string[], language: string) {
   const parser = new DOMParser();
   const html = parser.parseFromString(content, 'text/html');
-  const minLevel = Math.min(...levels.map((el) => Number(el)));
+  let minLevel = 1;
+  if (levels.length) {
+    minLevel = Math.min(...levels.map((el) => Number(el)));
+  } 
   let snippets: string[] = [] ;
   let snippetsFormatted: string[] = [];
   let keywords: string[] = []