permit explicitly empty qt.conf entries

permit explicitly empty qt.conf entries

this allows expressing "no SettingsPath" cleanly, and makes the
internals cleaner.

this is marginally behavior-incompatible in that if somebody had Prefix
or Data set to an empty string (rather than a dot, which is hinted at by
the documentation and commanded by common sense), he would have gotten
the binary's directory resp. Prefix, while now it will be just empty.

Change-Id: I3c11cc4e8671ffb604d4889389764a9ba9c2e7e6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>