I am getting to know SSIS, I apologize if the question is too simple.
I got a set of tasks inside a foreach-loop-container. T
I would create a For Loop Container around the task that needs the condition with the following conditions (@iis the loop counter, @foo is your user variable that you want to test):
For Loop Container
@i
@foo
@i=0
@i<1 && !ISNULL(@Foo) && @Foo!=""
@i=@i+1