QbsProject: Make initial CppTools::ProjectInfo valid

This fixes triggering the QTC_ASSERT in updateCppCompilerCallData() if
the user opens and builds the following qbs project:

    import qbs
    Product {}

Conceptually the initial ProjectInfo should stay invalid, but this would
result in a bigger / more risky change. Will be addressed in master
properly.

Change-Id: I74d3374b438884f38947f551915a80ec836a85c1
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>