Unify expectations on spec components folder

In this PR (https://github.com/consul/consul/pull/4683) a new syntax was introduced
in the component specs to check that the component was not rendering.
It seems interesting to add this syntax to the rest of the cases and thus unify the way
we check that a component is not rendering.

spec/components/shared/banner_component_spec.rb

@@ -82,13 +82,13 @@ describe Shared::BannerComponent do
 
       render_inline Shared::BannerComponent.new("debates")
 
-      expect(page).not_to have_css ".banner"
+      expect(page.native.inner_html).to be_empty
     end
   end
 
   it "does not render anything given nil" do
     render_inline Shared::BannerComponent.new(nil)
 
-    expect(page).not_to have_css ".banner"
+    expect(page.native.inner_html).to be_empty
   end
 end