feat(index): add conditional rendering for testimonials section

- Implemented a check to display the testimonials section only when there are testimonials available, enhancing the user experience by preventing empty sections.
- Updated the logic for loading testimonials from settings to handle cases where the testimonials list may be null, ensuring robustness in data handling.

These changes improve the layout and presentation of the Index page, providing users with relevant content based on available testimonials.

src/FrontOffice.Main/Pages/Index.razor

@@ -429,6 +429,8 @@ else
 @* ═══════════════════════════════════════════════
    6. TESTIMONIALS
    ═══════════════════════════════════════════════ *@
+@if (_testimonials.Count > 0)
+{
 <section class="section-landing">
     <MudContainer MaxWidth="MaxWidth.Large">
         <div class="text-center mb-8 fade-in-up">
@@ -465,6 +467,7 @@ else
         </MudGrid>
     </MudContainer>
 </section>
+}
 
 @* ═══════════════════════════════════════════════
    7. FAQ