Fix bug: Add nil verification to bitstream filename

lib/tasks/dbInfo.rake

@@ -30,11 +30,14 @@ namespace :dbinfo do
         break if learning_objects.empty?
 
         learning_objects.each do |lo|
-          bitstream_format = File.extname(lo.get_bitstream_filename_of "ORIGINAL")
-          unless bitstreams_formats_hash[bitstream_format].nil?
-            bitstreams_formats_hash[bitstream_format] += 1
-          else
-            bitstreams_formats_hash[bitstream_format] = 1
+          bitstream_filename = lo.get_bitstream_filename_of "ORIGINAL"
+          unless bitstream_filename.nil?
+            bitstream_format = File.extname(bitstream_filename)
+            unless bitstreams_formats_hash[bitstream_format].nil?
+              bitstreams_formats_hash[bitstream_format] += 1
+            else
+              bitstreams_formats_hash[bitstream_format] = 1
+            end
           end
         end