a!queryProcessAnalytics

Not sure whether you can have a task which is assigned to nobody. What do you want to achieve?

But besides that, this field is a list and that list can only be empty, but not null. I typically add a special boolean column to the report that does these kinds of logic.

Business request was to give a screen with ONLY assigned tasks on a grid from which they would like to select and reassign the tasks if the current assigned user goes on unplanned leaves.

I think you just need a filter that checks if the user is in the list. You could add a dropdown filter above the grid to allow selecting a user and apply the selected user to the filter like this:

a!queryFilter(
  field: "c8",
  operator: "in",
  value: local!selectedUser
)

I think you just need a filter that checks if the user is in the list. You could add a dropdown filter above the grid to allow selecting a user and apply the selected user to the filter like this:

a!queryFilter(
  field: "c8",
  operator: "in",
  value: local!selectedUser
)

Wanted to know if  operator = "is not null" on a!queryFilter while used in a!queryProcessAnalytics does not work?

So that we can make sure we do not take this approach before committing to business.

When you use query filters on process reports it's super finnicky about data types.  "is not null" strictly requires the data type of the target field to be single, plain, text.  If it's something else, the filter will just not work.

One suggestion I can make based on what I've had to do in the past: you can create extra columns in the process report, containing the same data but assigned differently.  For instance, if the definition for the column you're trying to query the NULLness of is defined as "tp!assignees", you could create a new column where you define it as "tostring(tp!assignees)", which would force the data in that (new) column to come back as singular, plain text data.