Avoid empty links in banners without content

This happened when previewing banners in the "new banner form", which
might cause accessibility issues when people access the list of links on
the page.

We were getting the following accessibility error:

```
link-name: Links must have discernible text (serious)
https://dequeuniversity.com/rules/axe/4.9/link-name?application=axeAPI

The following node violate this rule:

  Selector: a[href$="new"]
  HTML: <a href="/admin/banners/new"><h2></h2><h3></h3></a>
  Fix all of the following:
  - Element is in tab order and does not have accessible text
  Fix any of the following:
  - Element does not have text that is visible to screen readers
  - aria-label attribute does not exist or is empty
  - aria-labelledby attribute does not exist, references elements that
    do not exist or references elements that
  - Element has no title attribute
```

app/components/shared/banner_component.html.erb

@@ -1,5 +1,3 @@
-<% if banner %>
-  <div class="banner" style="background-color:<%= banner.background_color %>;">
-    <%= sanitize link, attributes: %w[href style] %>
-  </div>
-<% end %>
+<div class="banner" style="background-color:<%= banner.background_color %>;">
+  <%= sanitize link, attributes: %w[href style] %>
+</div>

app/components/shared/banner_component.rb

@@ -13,6 +13,10 @@ class Shared::BannerComponent < ApplicationComponent
                 end
   end
 
+  def render?
+    banner && (banner.title.present? || banner.description.present?)
+  end
+
   private
 
     def link

spec/components/shared/banner_component_spec.rb

@@ -31,6 +31,14 @@ describe Shared::BannerComponent do
     expect(page).not_to have_content "Proposal banner"
   end
 
+  it "does not render an empty banner" do
+    banner = build(:banner, title: "", description: "")
+
+    render_inline Shared::BannerComponent.new(banner)
+
+    expect(page).not_to be_rendered
+  end
+
   context "several banners available in the same section" do
     before do
       create(:banner,