Yes, you can make the legend ignore blank data series fields in your Pie Chart. Here's how:

1. Select the Pie Chart by clicking on it.
2. Click on the "Chart Elements" button (plus sign icon) that appears next to the chart.
3. Check the box next to "Legend" to add the legend to your chart if it's not already there.
4. Right-click on the legend and select "Format Legend" from the drop-down menu.
5. In the "Format Legend" pane that appears on the right-hand side of the screen, click on the "Legend Options" tab.
6. Under "Label Contains", select "Value From Cells" from the drop-down menu.
7. Click on the "Select Data..." button.
8. In the "Select Data Source" dialog box that appears, click on the "Hidden and Empty Cells" button.
9. In the "Hidden and Empty Cell Settings" dialog box, select the "Show data in hidden rows and columns" option.
10. Click "OK" to close all the dialog boxes.

Now, the legend in your Pie Chart will only show the data series names that have values, and will ignore the blank data series fields.