Don't bother going through an entire Banzai pipeline for empty text

This bails out of `cacheless_render` immediately unless the provided
text is present, since there's no point.

This is a slight improvement in our test performance. Across the
creation of 1,000 `Namespace` records, which caches the `description`
field and which is blank by default in its factory, this saves about
four seconds, which... sure. Why not.

lib/banzai/renderer.rb

@@ -132,6 +132,8 @@ module Banzai
     end
 
     def self.cacheless_render(text, context = {})
+      return text.to_s unless text.present?
+
       Gitlab::Metrics.measure(:banzai_cacheless_render) do
         result = render_result(text, context)