Uncategorized tools would no longer be configurable after first run.

Avoid writing empty string prefixed arrays into settings, and some
isNull vs isEmpty confusion (xml reader seems to read null instead of
empty strings in some cases).

src/plugins/coreplugin/dialogs/externaltoolconfig.cpp

@@ -179,8 +179,9 @@ QModelIndex ExternalToolModel::index(int row, int column, const QModelIndex &par
                 return createIndex(row, 0, items.at(row));
             }
         }
-    } else if (column == 0 && row < m_tools.keys().count())
+    } else if (column == 0 && row < m_tools.keys().count()) {
         return createIndex(row, 0);
+    }
     return QModelIndex();
 }