Depending on the circumstances there are 2 general ways to hide Null values in Tableau.
1. Format the pill and hide the null values
Select Hide in the Special Values section of the pane. For further explanation I have written about this previously.
2. Drag the pill into the filter section and hide null values using the Special section. In the Special section of the filter you will see Not Null. Select that and it will filter out the null values although this could inadvertently hide some things you want to see – for example of you’re filtering a chart axis.