fix: guard filter snippet include when no form passed to metrics template (#14945)

critical_product_metrics view renders metrics.html without a form context
variable. Django resolves undefined template vars as empty string, causing
get_filter_groups to crash with AttributeError on str.visible_fields().

Wrapping the filter_snippet include in {% if form %} prevents the crash.
Fixes #14944.

Author: valentijnscholten

dojo/templates/dojo/metrics.html

@@ -147,9 +147,11 @@ <h3 {% if not critical_prods %}class="has-filters" {% endif %}>
                         </h3>
                     </div>
                 </div>
+                {% if form %}
                 <div id="the-filters" class="is-filters panel-body collapse">
                     {% include "dojo/filter_snippet.html" with form=form clear_link="/metrics/product/type" %}
                 </div>
+                {% endif %}
             </div>
         </div>
     </div>

dojo/templates_classic/dojo/metrics.html

@@ -155,9 +155,11 @@ <h3 {% if not critical_prods %}class="has-filters" {% endif %}>
                         </h3>
                     </div>
                 </div>
+                {% if form %}
                 <div id="the-filters" class="is-filters panel-body collapse">
                     {% include "dojo/filter_snippet.html" with form=form clear_link="/metrics/product/type" %}
                 </div>
+                {% endif %}
             </div>
         </div>
     </div>