you can do check on 'HasContents’ boolean of the file image if its true don't show if false show it
hope this answers your question
In the Dataveiw page Add two containers and add visibilty to the container since file dropper widegt doesnot have visibility Condition by default. You can use the idea given by Mahesh for the visibility condition.
Hope this helps!!
If the association to your entity(file document generalized entity) from your main entity is empty then show the file dropper widget if not hide it.