base: Fix jinja2.exceptions.UndefinedError on Python ≥ 3.7.

Python 3.7 changed dict.update to avoid swallowing exceptions when
checking for the keys attribute (https://bugs.python.org/issue31572).
This broke {}.update(jinja2.Undefined()).

Fix it with an explicit default.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

templates/zerver/base.html

@@ -34,7 +34,7 @@
         {% endblock %}
 
         {% set all_page_params = default_page_params.copy() %}
-        {% set _ = all_page_params.update(page_params) %}
+        {% set _ = all_page_params.update(page_params|default({})) %}
         <div hidden id="page-params" data-params='{{ all_page_params|tojson }}'></div>
     </body>