Prevent exception on empty URL in markdown text

An empty URL part in markdown text leads to an exception
in the mardown renderer. When a comment
includes such an empty URL string, it ends up with a
code 500. This happens when people post code examples
which contain the same syntax as a markdown link.
Instead of throwing an exception, we can just render
the original content incase of an empty URL part.

Kudos to @eduardoj

Fixes #10259

src/api/lib/obsapi/markdown_renderer.rb

@@ -30,6 +30,10 @@ def block_html(raw_html)
 
     # unfortunately we can't call super (into C) - see vmg/redcarpet#51
     def link(link, title, content)
+      # A return value of nil will not output any data
+      # the contents of the span will be copied verbatim
+      return nil if link.blank?
+
       title = " title='#{title}'" if title.present?
       begin
         link = URI.join(::Configuration.obs_url, link)

src/api/spec/helpers/webui/markdown_helper_spec.rb

@@ -62,5 +62,11 @@
         "<p><a href=\"https://build.opensuse.org\">&amp;lt;script&amp;gt;&amp;lt;/script&amp;gt;</a></p>\n"
       )
     end
+
+    it 'just returns the original content on empty URIs' do
+      expect(render_as_markdown('installed_ver = self.core.version_func[deps_info[6]]()')).to eq(
+        "<p>installed_ver = self.core.version_func[deps_info[6]]()</p>\n"
+      )
+    end
   end
 end