CMake: Fix changing build directory

It wasn't possible to change the build directory if the old directory
didn't already contain a build.

Amends 7e6e02eb08

Change-Id: I741f6981273a4e04cceabde5b15b5ad951535ef8
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.cpp

@@ -121,8 +121,9 @@ CMakeBuildConfiguration::CMakeBuildConfiguration(Target *target, Core::Id id)
                     == QMessageBox::Ok) {
                     return newDir;
                 }
+                return Utils::nullopt;
             }
-            return Utils::nullopt;
+            return newDir;
         });
 
     addAspect<InitialCMakeArgumentsAspect>();