Remove the need to call #presence to show a fall-back HTML

Right now, the #paginate method doesn't show anything when there's only one
page. While this is expected, some times there's a case where showing a fall
back message is appropriate. In order to do so, a subsequent #presence call is
needed because the #paginate method always returns a string. This commit adds a
 #presence call to the #render method, so it'll be a little cleaner to show a
fall-back message.

  related: https://github.com/kaminari/kaminari/pull/785

kaminari-core/lib/kaminari/helpers/paginator.rb

@@ -26,7 +26,11 @@ module Kaminari
       # render given block as a view template
       def render(&block)
         instance_eval(&block) if @options[:total_pages] > 1
-        @output_buffer
+
+        # This allows for showing fall-back HTML when there's only one page:
+        #
+        #   <%= paginate(@search_results) || "Showing all search results" %>
+        @output_buffer.presence
       end
 
       # enumerate each page providing PageProxy object as the block parameter