Don't sanitize anchor elements with no href (#16667)

* If href is nil, allow node

* Add unit test showing no change

My first pass had the `<a>` within an h1 content, but the scrubber
added a newline after the closing a tag?

app/sanitizers/feed_markdown_scrubber.rb

@@ -8,6 +8,6 @@ class FeedMarkdownScrubber < Rails::Html::PermitScrubber
   def allowed_node?(node)
     return true if tags.include?(node.name) && node.name != "a"
 
-    node.name == "a" && !node["href"].start_with?("#")
+    node.name == "a" && !node["href"]&.start_with?("#")
   end
 end

spec/sanitizers/feed_markdown_scrubber_spec.rb

@@ -57,4 +57,10 @@ RSpec.describe FeedMarkdownScrubber, type: :permit_scrubber do
     clean = sanitize(bad_html, scrubber: described_class.new)
     expect(clean).to eq(good_html)
   end
+
+  it "does not scrub anchors with no link" do
+    good_html = "I put an anchor <a>here</a>"
+    clean = sanitize(good_html, scrubber: described_class.new)
+    expect(clean).to eq(good_html)
+  end
 end