fix(base template): check for existence of request to show user menu

A request object is not sent to the `BadRequest` exception, so we check whether such an object exists before trying to render the `400.html` template.

django/cantusdb_project/templates/base.html

@@ -285,7 +285,7 @@ <h5>Contact Us</h5>
                             </div>
                         </div>
                         <div class="col-md-4" id="login">
-                            {% if not request.user.is_anonymous %}
+                            {% if request and not request.user.is_anonymous %}
                                 {% if request.user|in_groups:"contributor,editor,project manager" %}
                                     <h5>Admin Navigation</h5>
                                     <ul class="list-group list-group-flush">