LayoutBuilder: Fix assert

Let's add an explicit QLabel in the BaseBoolAspect, as the layout
builder does not expect an empty string in the middle of a row.

Change-Id: I19ef80d31ad1f2527af7e5677751922ae4d5fcff
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/projectconfigurationaspects.cpp

@@ -436,7 +436,7 @@ void BaseBoolAspect::addToLayout(LayoutBuilder &builder)
     d->m_checkBox = new QCheckBox();
     if (d->m_labelPlacement == LabelPlacement::AtCheckBox) {
         d->m_checkBox->setText(d->m_label);
-        builder.addItem(QString());
+        builder.addItem(new QLabel);
     } else {
         builder.addItem(d->m_label);
     }