docs: tutorial: add verify to avoid empty content for newTodo (#2712)

Author: niceplugin

src/tutorial/src/step-7/App/template.html

@@ -1,5 +1,5 @@
 <form @submit.prevent="addTodo">
-  <input v-model="newTodo">
+  <input v-model="newTodo" required placeholder="new todo">
   <button>할 일 추가</button>
 </form>
 <ul>

src/tutorial/src/step-8/App/template.html

@@ -1,5 +1,5 @@
 <form @submit.prevent="addTodo">
-  <input v-model="newTodo">
+  <input v-model="newTodo" required placeholder="new todo">
   <button>Add Todo</button>
 </form>
 <ul>

src/tutorial/src/step-8/_hint/App/template.html

@@ -1,5 +1,5 @@
 <form @submit.prevent="addTodo">
-  <input v-model="newTodo">
+  <input v-model="newTodo" required placeholder="new todo">
   <button>Add Todo</button>
 </form>
 <ul>