FIX: s3 cdn would break cooking if <img> tag had no src

lib/pretty_text.rb

@@ -224,6 +224,7 @@ module PrettyText
 
   def self.add_s3_cdn(doc)
     doc.css("img").each do |img|
+      next unless img["src"]
       img["src"] = img["src"].sub(Discourse.store.absolute_base_url, SiteSetting.s3_cdn_url)
     end
   end

spec/components/pretty_text_spec.rb

@@ -323,8 +323,9 @@ describe PrettyText do
     SiteSetting.s3_upload_bucket = "test"
     SiteSetting.s3_cdn_url = "https://awesome.cdn"
 
-    raw = "<img src='#{Discourse.store.absolute_base_url}/original/9/9/99c9384b8b6d87f8509f8395571bc7512ca3cad1.jpg'"
-    cooked = "<p><img src='https://awesome.cdn/original/9/9/99c9384b8b6d87f8509f8395571bc7512ca3cad1.jpg'></p>"
+    # add extra img tag to ensure it does not blow up
+    raw = "<img><img src='#{Discourse.store.absolute_base_url}/original/9/9/99c9384b8b6d87f8509f8395571bc7512ca3cad1.jpg'"
+    cooked = "<p><img><img src='https://awesome.cdn/original/9/9/99c9384b8b6d87f8509f8395571bc7512ca3cad1.jpg'></p>"
 
     expect(PrettyText.cook(raw)).to match_html(cooked)
   end