Don't show unknown tier list if there are none

Signed-off-by: Alex Saveau <[email protected]>

Author: SUPERCILEX

template.html

@@ -80,6 +80,7 @@ <h3 class="text-center">{{tier_info.0}}</h3>
                 {{/each}}
                 </div>
             </div>
+            {{#if additional.tiers.unknown_tier}}
             <h2 class="text-center">Unknown tier targets</h2>
             <div class="container">
                 <div class="row">
@@ -92,6 +93,7 @@ <h2 class="text-center">Unknown tier targets</h2>
                     </div>
                 </div>
             </div>
+            {{/if}}
             <div class="card-footer text-muted bg-transparent text-center">
                 <p>Generated at {{additional.datetime}}</p>
                 <p>Contributions are <a href="https://github.com/rust-lang/rustup-components-history">welcome</a>!