[FIX] web: no traceback on button call action of type 'button'.

Before this commit, there was a traceback when clicking on a button
that generates a button call action with type `'button'`.

This is due to the handler of such an action that assumes only a small
set of button actions. Each of those cases correctly sets the local
variable `def` to a deferred, so that the post-processing is correct
(i.e. it handles this variable as a deferred). However, this is wrong
with a different button call action, so that this variable is `undefined`
instead of a deferred.

This commit fixes the issue by ensuring that the variable `def` is always
a deferred. As a result, the post-processing can make the correct assumption
that this variable is a deferred.

Steps to reproduce:

 - Install Google Drive Integration app
 - Go to General Settings
 - In the category "Google Drive", click on "Get Authorization Code"
   (displayed as "Get Author...")

Task-ID 1861630

addons/web/static/src/js/chrome/action_manager_act_window.js

@@ -663,7 +663,7 @@ ActionManager.include({
         var env = ev.data.env;
         var context = new Context(env.context, actionData.context || {});
         var recordID = env.currentID || null; // pyeval handles null value, not undefined
-        var def;
+        var def = $.Deferred();
 
         // determine the action to execute according to the actionData
         if (actionData.special) {