code: def _build_param_string(params):
          
          pairs = []
          for key, value in params.iteritems():
              if value is None:
                  value = ''
              pairs.append('{0}={1}'.format(key, value))
          if len(pairs) > 0:
              return '?{0}'.format('&'.join(pairs))
          return ''
docs: Build query params string from a dictionary.
  
          Args:
              params (dict): A dictionary of params
  
          Returns:
              string: A valid url query params string.