FIX: Video thumbnails for missing videos (#28152)

Skip trying to find a thumbnail if the video src cannot be found.

Bug report: https://meta.discourse.org/t/317423

lib/pretty_text.rb

@@ -452,6 +452,7 @@ module PrettyText
       .css(".video-placeholder-container")
       .each do |video|
         video_src = video["data-video-src"]
+        next if video_src == "/404" || video_src.nil?
         video_sha1 = File.basename(video_src, File.extname(video_src))
         thumbnail = Upload.where("original_filename LIKE ?", "#{video_sha1}.%").last
         if thumbnail

spec/lib/pretty_text_spec.rb

@@ -2794,4 +2794,42 @@ HTML
       require("discourse-common/lib/deprecated").default("Some deprecation message");
     JS
   end
+
+  describe "video thumbnails" do
+    before do
+      SiteSetting.authorized_extensions = "mp4|png"
+      @video_upload = Fabricate(:upload, original_filename: "video.mp4", extension: "mp4")
+    end
+
+    after { Upload.where(original_filename: ["404.png", "#{@video_upload.sha1}.png"]).destroy_all }
+
+    it "does not link to a thumbnail image if the video source is missing" do
+      Fabricate(:upload, original_filename: "404.png", extension: "png")
+
+      html = <<~HTML
+          <p></p><div class="video-placeholder-container" data-video-src="/404"></div><p></p>
+        HTML
+      doc = Nokogiri::HTML5.fragment(html)
+      described_class.add_video_placeholder_image(doc)
+
+      expect(doc.to_html).to eq(html)
+    end
+
+    it "links to a thumbnail image if the video source is valid" do
+      thumbnail =
+        Fabricate(:upload, original_filename: "#{@video_upload.sha1}.png", extension: "png")
+
+      html = <<~HTML
+        <p></p><div class="video-placeholder-container" data-video-src="#{@video_upload.url}"></div><p></p>
+      HTML
+      doc = Nokogiri::HTML5.fragment(html)
+      described_class.add_video_placeholder_image(doc)
+
+      html_with_thumbnail = <<~HTML
+        <p></p><div class="video-placeholder-container" data-video-src="#{@video_upload.url}" data-thumbnail-src="http://test.localhost#{thumbnail.url}"></div><p></p>
+      HTML
+
+      expect(doc.to_html).to eq(html_with_thumbnail)
+    end
+  end
 end