CMakePM: Filter out empty key configuration items

There were cases when an empty "-D" CMake command line parameter would
be genered, resulting in a failed CMake project configuration.

Task-number: QTCREATORBUG-27256
Fixes: QTCREATORBUG-27237
Change-Id: Ic26c9dfe72db154cd76fdcc913eb7fe623b3123c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.cpp

@@ -619,7 +619,7 @@ void CMakeBuildSettingsWidget::updateInitialCMakeArguments()
             *it = ci;
             if (ci.isUnset)
                 initialList.erase(it);
-        } else {
+        } else if (!ci.key.isEmpty()) {
             initialList.push_back(ci);
         }
     }