Making Net_SFTP::chdir capable of handling the empty string.

Fixing a ugly notice when using the empty string as parameter for chdir:
  Notice: Uninitialized string offset: -1 in Net/SFTP.php line 617
Making Net_SFTP::chdir capable of handling the empty string.

Updated patch after hint from bantu.
Merge pull request #173 from h4cc/patch-1

Making Net_SFTP::chdir capable of handling the empty string.