FIX: blank cooked markdown could raise an exception in logs

Previously if somehow a user created a blank markdown document using tag
tricks (eg `<p></p><p></p><p></p><p></p><p></p><p></p>`) and so on, we would
completely strip the document down to blank on post process due to onebox
hack.

Needs a followup cause I am still unclear about the reason for empty p stripping
and it can cause some unclear cases when we re-cook posts.

lib/cooked_post_processor.rb

@@ -23,7 +23,8 @@ class CookedPostProcessor
     @cooking_options[:topic_id] = post.topic_id
     @cooking_options = @cooking_options.symbolize_keys
 
-    @doc = Nokogiri::HTML::fragment(post.cook(post.raw, @cooking_options))
+    cooked = post.cook(post.raw, @cooking_options)
+    @doc = Nokogiri::HTML::fragment(cooked)
     @has_oneboxes = post.post_analyzer.found_oneboxes?
     @size_cache = {}
 

lib/oneboxer.rb

@@ -116,7 +116,11 @@ module Oneboxer
     end
 
     # strip empty <p> elements
-    doc.css("p").each { |p| p.remove if p.children.empty? }
+    doc.css("p").each do |p|
+      if p.children.empty? && doc.children.count > 1
+        p.remove
+      end
+    end
 
     Result.new(doc, changed)
   end

spec/components/cooked_post_processor_spec.rb

@@ -1783,6 +1783,13 @@ describe CookedPostProcessor do
       expect(reply.raw).to eq(raw)
     end
 
+    it "does not generate a blank HTML document" do
+      post = Fabricate(:post, topic: topic, raw: "<sunday><monday>")
+      cp = CookedPostProcessor.new(post)
+      cp.post_process
+      expect(cp.html).to eq("<p></p>")
+    end
+
     it "works only on new posts" do
       Fabricate(:post, topic: topic, hidden: true, raw: "this is the second post after")
       Fabricate(:post, topic: topic, post_type: Post.types[:small_action])