Initialize error_message to empty string avoid string op errors

Update template branching to account for empty content template output

Author: Omripresent

netbox/extras/views.py

@@ -966,7 +966,7 @@ def get_extra_context(self, request, instance):
 
         # Render the config template
         rendered_config = None
-        error_message = None
+        error_message = ''
         if config_template := instance.get_config_template():
             try:
                 rendered_config = config_template.render(context=context_data)

netbox/templates/extras/object_render_config.html

@@ -63,11 +63,15 @@ <h2 class="card-header d-flex justify-content-between">
             </h2>
             <pre class="card-body" id="rendered_config">{{ rendered_config }}</pre>
           </div>
-        {% else %}
+        {% elif error_message %}
           <div class="alert alert-warning">
             <h4 class="alert-title mb-1">{% trans "Error rendering template" %}</h4>
             {% trans error_message %}
           </div>
+        {% else %}
+          <div class="alert alert-warning">
+            <h4 class="alert-title mb-1">{% trans "Template output is empty" %}</h4>
+          </div>
         {% endif %}
       {% else %}
         <div class="alert alert-info">