Log a warning when the slug is empty (#7357)

Merge pull request 7357

lib/jekyll/utils.rb

@@ -222,6 +222,7 @@ module Jekyll
       slug.gsub!(%r!^\-|\-$!i, "")
 
       slug.downcase! unless cased
+      Jekyll.logger.warn("Warning:", "Empty `slug` generated for '#{string}'.") if slug.empty?
       slug
     end
 

test/test_utils.rb

@@ -282,6 +282,11 @@ class TestUtils < JekyllUnitTest
         Utils.slugify("The _config.yml file?", :mode => "none", :cased => true)
       )
     end
+
+    should "records a warning in the log if the returned slug is empty" do
+      expect(Jekyll.logger).to receive(:warn)
+      assert_equal "", Utils.slugify("💎")
+    end
   end
 
   context "The \`Utils.titleize_slug\` method" do