[libqtsparql] Fix invalid URL error in EndpointDriver
[libqtsparql] Fix invalid URL error in EndpointDriver

Only fails with Qt5.  The error is "Path component is relative and
authority is present".  With Qt5, toString()/toEncoded() called on a
not isValid() QUrl instance returns empty string.  As a result an
invalid HTTP Request-Line with empty Request-URI ("GET  HTTP/1.0") is
sent.

Detected by tst_qsparql_endpoint.

vranki/libqtsparql,vranki/libqtsparql,vranki/libqtsparql,vranki/libqtsparql