DEV: Correct handling of empty Accept headers

lib/discourse_activity_pub/json_ld.rb

@@ -75,6 +75,7 @@ def valid_content_type?(value)
     end
 
     def valid_accept?(value)
+      return false if value.blank?
       value.split(",").compact.collect(&:strip).all? { |v| valid_content_type?(v) }
     end
 

spec/lib/discourse_activity_pub/json_ld_spec.rb

@@ -45,6 +45,10 @@
         false,
       )
     end
+
+    it "does not validate empty accept headers" do
+      expect(described_class.valid_accept?(nil)).to eq(false)
+    end
   end
 
   describe "#address_json" do