Hello @Omar, welcome to the Quick Sight Community!
I was able to put together a control that will allow your user to include or exclude NULL values.
First create a string value parameter, I called it includeNulls. Make the default Yes or No depending on what your requirements are and create a control. I made the control a dropdown with the options Yes and No, then pinned it to the top of the sheet.
Now create a calculated field that will use your field that is returning the team values or null. It will look something like this:
ifelse(isNull({teams}) AND ${includeNulls} = "Yes", NULL, isNull({teams}) AND ${includeNulls} = "No", "NULL", {teams})
Now depending on that parameter value, you will either be returning an actual NULL or a string value of “NULL”. On your visual, replace the field in question with this new calculated field, I’ll call it filteredTeams. Create a filter for filteredTeams, set it to a custom filter and make sure your dropdown is set to Include Nulls. Set the filter condition to Does not equal and write NULL in the text box. Once you apply this, your control should manage whether the nulls are displayed or not!
I will mark this as a solution for now, but if you have any follow-up questions please let me know!