Write empty CSV file when input data is an empty array

lib/petrify.rb

@@ -29,14 +29,19 @@ module Petrify
     # https://support.google.com/webmasters/answer/183668?hl=en&ref_topic=4581190
   end
 
-  # Write a CSV file
+  # Write a CSV file from an array of hashes (or an empty array)
   def self.csv(path_items, filename, data)
     dir = create_path(path_items)
     fn = File.join(dir, filename + '.csv')
     
-    csv_string = CSV.generate do |csv|
-      csv << data.first.keys # header row
-      data.each { |row| csv << row.values }
+    if data.size > 0
+      csv_string = CSV.generate do |csv|
+        csv << data.first.keys # header row
+        data.each { |row| csv << row.values }
+      end
+    else
+      # There's no data so write an empty file (no header row either)
+      csv_string = ''
     end
     
     File.write(fn, csv_string)