FIX: Do not try to show logo if it does not exist (#12968)

The generator tried to include the logo in the certificate, even when
it did not exist and that caused a server error.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/templates/new_user.svg.erb

@@ -457,9 +457,11 @@
     <text x="<%= @width / 2 %>" y="240.94" text-anchor="middle" font-size="24" fill="#020403" font-family="Tangerine, Tangerine">
       <%= @name %>
     </text>
-    <g transform="translate(<%= @logo_group[:width] / 2 - (@logo_group[:size] / 2) %> <%= @logo_group[:height] %>)">
-      <image height="<%= @logo_group[:size] %>px" width="<%= @logo_group[:size] %>px" href="<%= @logo_group[:logo_uri] %>"/>
-    </g>
+    <% if @logo_group.present? %>
+      <g transform="translate(<%= @logo_group[:width] / 2 - (@logo_group[:size] / 2) %> <%= @logo_group[:height] %>)">
+        <image height="<%= @logo_group[:size] %>px" width="<%= @logo_group[:size] %>px" href="<%= @logo_group[:logo_uri] %>"/>
+      </g>
+    <% end %>
   </g>
   <g id="Layer_1-2" data-name="Layer 1">
     <path d="M339.328 47.187c-.034 1.193 3.25 2.2 6.51 3.383 5.5 1.994 13.1 3.73 17.68 3.112 5.05-.55 4.982-2.217 4.054-4.8-.1-4.647.258-9.493.156-14.142l-27.128-.458c.5 4.698-1.778 8.207-1.272 12.905z" transform="translate(-.108)" fill-rule="evenodd" fill="url(#GradientFill_24)"/>