function nullIfAbsent(matchPart) {
  return ('string' == typeof matchPart) && (matchPart.length > 0)
         ? matchPart
         : null;
}
returns the first value for a given cgi parameter or null if the given
parameter name does not appear in the query string.
If the given parameter name does appear, but has no '<tt>=</tt>' following
it, then the empty string will be returned.
@return {string|null}