Use an empty QString (instead of NULL) when RdConfig->tempDirectory() isn't defined
Merge pull request #19 from tryphon/rdconf-conf-temp-directory-empty

Empty QString instead of NULL for RdConfig->tempDirectory()

Indeed -- in certain contexts --i.e. casting to 'const char *' -- a NULL ends up evaluating to a "(null)" string, while QString::isEmpty() will also handle the NULL case.

Merged -- thanks!