Remove unecessary null check

Author: joaosimbiose

lib/rbslicer/utils/validators.rb

@@ -195,9 +195,6 @@ def valid_keys?
           if !value.is_a?(Array)
             raise Exceptions::InvalidQueryException, 'The key \'order\' in '\
                                                     'query has a invalid value.'
-          elsif value.nil?
-            raise Exceptions::InvalidQueryException, 'The key \'order\' in '\
-                                                    'query has a null value.'
           elsif value.empty?
             raise Exceptions::InvalidQueryException, 'The key \'order\' in '\
                                                     'query has a empty array.'