Add strict_variables/strict_filters render options to check for undefined variables and filters

lib/liquid/block_body.rb

@@ -76,6 +76,9 @@ module Liquid
           end
         rescue MemoryError => e
           raise e
+        rescue UndefinedVariable, UndefinedDropMethod, UndefinedFilter => e
+          context.handle_error(e, token.line_number)
+          output << nil
         rescue ::StandardError => e
           output << context.handle_error(e, token.line_number)
         end