Update test string comparison method

Author: ronaldtse

lib/interscript.rb

@@ -64,7 +64,7 @@ def transliterate(system_code, string)
         end
       end
 
-      output.unicode_normalize
+      output ? output.unicode_normalize : output
     end
 
     private

spec/interscript_spec.rb

@@ -9,8 +9,10 @@
 
       system["tests"]&.reduce([]) do |testresults, test|
         it "test for #{test}" do
-          result = Interscript.transliterate system_name, test["source"]
-          expect(result.unicode_normalize).to eq(test["expected"].unicode_normalize)
+          result = Interscript.transliterate(system_name, test["source"]) || ""
+          expected = (test["expected"] || "").unicode_normalize
+
+          expect(result).to eq(expected)
         end
       end
     end