Returning QString() from here for unset vc will result in the
substitution to fail and thus will leave "%{VersionControl}" as
the text.

Returning a empty non-Null string will make turn the replacement
into "" instead, which is what is expected.

Change-Id: I9dc890deb5f9ea1ac28c1546e4c1bce804d196ef
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>