[netinstall] Sanitize PackageTreeItem::data()

- Use of != nullptr for QString just a bad idea
- Massage code so structure for packages resembles that of groups

src/modules/netinstall/PackageTreeItem.cpp

@@ -108,14 +108,18 @@ PackageTreeItem::row() const
 QVariant
 PackageTreeItem::data( int column ) const
 {
-    if ( packageName() != nullptr )  // package
+    if ( !packageName().isEmpty() )  // packages have a packagename, groups don't
     {
-        if ( !column )
+        switch ( column )
         {
+        case 0:
             return QVariant( packageName() );
-        }
+        default:
             return QVariant();
         }
+    }
+    else
+    {
         switch ( column )  // group
         {
         case 0:
@@ -126,6 +130,7 @@ PackageTreeItem::data( int column ) const
             return QVariant();
         }
     }
+}
 
 PackageTreeItem*
 PackageTreeItem::parentItem()