Allow the binlog_path to be set to an empty string

I've recently had trouble with Sphinx failing to start with the following error: 

    FATAL: binlog: log open error: failed to open /srv/ebth-com/shared/tmp/binlog/production/binlog.001: No such file or directory

It appears that Sphinx sees the binlog.meta and binlog.lock files and thinks that there should be a binlog.001 file. According to the [Sphinx documentation](http://sphinxsearch.com/docs/archives/1.10/conf-binlog-path.html), the `binlog_path` is only used for realtime indexing, and in order to disable it, set `binlog_path` to an empty string in the Sphinx conf file.

However, ThinkingSphinx doesn't take a blank string into account when trying to create the directory as a part of the configure process. I simply added a check to the `render_to_file` method.

Thanks to Bryan @railsmachine for helping to diagnose the issue.