display the 'add password' button only when the input field isn't empty

app/templates/uploadPassword.js

@@ -11,16 +11,30 @@ module.exports = function(state, emit) {
     </div>
     <form class="setPassword hidden" onsubmit=${setPassword}>
       <input id="unlock-input"
+        class="unlock-input input-no-btn"
         maxlength="64"
         autocomplete="off"
-        placeholder="${state.translate('unlockInputPlaceholder')}"/>
+        placeholder="${state.translate('unlockInputPlaceholder')}"
+        oninput=${inputChanged}/>
       <input type="submit"
         id="unlock-btn"
-        class="btn"
+        class="btn btn-hidden"
         value="${state.translate('addPasswordButton')}"/>
     </form>
   </div>`;
 
+  function inputChanged() {
+    const input = document.getElementById('unlock-input');
+    const btn = document.getElementById('unlock-btn');
+    if (input.value.length > 0) {
+      btn.classList.remove('btn-hidden');
+      input.classList.remove('input-no-btn');
+    } else {
+      btn.classList.add('btn-hidden');
+      input.classList.add('input-no-btn');
+    }
+  }
+
   function togglePasswordInput(e) {
     const unlockInput = document.getElementById('unlock-input');
     const boxChecked = e.target.checked;
@@ -36,6 +50,7 @@ module.exports = function(state, emit) {
     } else {
       unlockInput.value = '';
     }
+    inputChanged();
   }
 
   function setPassword(event) {