If there is no working directory explicitly set inside the run
configuration we end up using the current working directory of
Qt Creator.
We normally get an empty string if not having modified the respective
target ourselves although you can see a default working directory when
opening the respective target for the first time.

Task-number: QTCREATORBUG-16715
Change-Id: I6e16fd4f9b15759793653a6c229c44a1be2b7739
Reviewed-by: David Schulz <david.schulz@qt.io>