[FIX] web: CrashManager can crash if trying to show an empty message

If `d.message` is an empty string, `!!d.message` is false and the
crashmanager will try to display `d.error.data.message`… which does
not exist as it's a warning with an empty message not an upstream error.

That causes the crashmanager itself to crash, and even the warning
name/title to be lost.

addons/web/static/src/xml/base.xml

@@ -22,7 +22,7 @@
 <t t-name="CrashManager.warning">
     <div class="o_dialog_warning">
         <t t-js="d">
-            var message = d.message ? d.message : d.error.data.message;
+            var message = (d.message !== undefined) ? d.message : d.error.data.message;
             d.html_error = context.engine.tools.html_escape(message).replace(/\n/g, '<br/>');
         </t>
         <t t-raw="html_error"/>