The treemap component is able to handle negative colour intensities …. as long at least 1 intensity is positive. This means you need to add at least one row with an empty label field, 0 as value and 1% as intensity. As the label and value are both empty, it will not show up in the treemap itself.
This workaround is however not a solution in case of negative values. A negative value will simply not display. In most cases a simple warning message will suffice as it is probably an exception. In case it is not an exception you should advise to use another chart type for analysis. Like with pie charts, where negative values cannot be displayed.