emails: Show preheader block only if preheader is present.

templates/zerver/emails/email_base_messages.html

@@ -4,6 +4,7 @@
         <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
         <title>Zulip</title>
     </head>
+    {% if has_preheader %}
     <span style="display:none !important;
       visibility:hidden;
       color:transparent;
@@ -19,6 +20,7 @@
       overflow:hidden;">
         {% block preheader %}{% endblock %}
     </span>
+    {% endif %}
     {% block content %}{% endblock %}
     {% block manage_preferences %}{% endblock %}
 </html>

version.py

@@ -26,4 +26,4 @@ LATEST_RELEASE_ANNOUNCEMENT = "https://blog.zulip.org/2019/03/01/zulip-2-0-relea
 #   historical commits sharing the same major version, in which case a
 #   minor version bump suffices.
 
-PROVISION_VERSION = '49.0'
+PROVISION_VERSION = '49.1'

zerver/lib/digest.py

@@ -202,6 +202,8 @@ def handle_digest_email(user_profile_id: int, cutoff: float,
     context["new_streams"] = new_streams
     context["new_streams_count"] = new_streams_count
 
+    # TODO: Set has_preheader if we want to include a preheader.
+
     if render_to_web:
         return context