Return a `nil` rather than `false` stopper

With the latest Xapian (1.2.15.1) a `false` stopper raises:

```
ArgumentError: Wrong arguments for overloaded method 'QueryParser.stopper='.
Possible C/C++ prototypes are:
    void QueryParser.stopper=(Xapian::Stopper const *stop)
    void QueryParser.stopper=()

	from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/bundler/gems/xapian-fu-23859675dcbc/lib/xapian_fu/query_parser.rb:100:in `stopper='
	from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/bundler/gems/xapian-fu-23859675dcbc/lib/xapian_fu/query_parser.rb:100:in `query_parser'
	from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/bundler/gems/xapian-fu-23859675dcbc/lib/xapian_fu/query_parser.rb:84:in `parse_query'
	from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/bundler/gems/xapian-fu-23859675dcbc/lib/xapian_fu/xapian_db.rb:270:in `search'
```