I'm not aware of a way to do it within an Automator application, but give this a try - the script in the following example workflow toggles the enabled property of the action following it. Create a new workflow with 3 actions:
1) an Ask for Text action to get some input;
2) a Run AppleScript action to test the input and do something:
on run {input, parameters}
if (input as text) is "" then -- if no input then disable the following action
set currentAction to index of current action of front workflow -- the most recent completed action
tell Automator action index (currentAction + 2) of front workflow to set enabled to not enabled
end if
return input
end run
3) an Ask for Confirmation action to put up a dialog (or not).
You can use other action properties such as the name, but the index or id works better if there are more than one of the same action.