Hi,
Could you try something like this in your workflow:
- Add a return parameter to your workflow with a value that errors out if the export rule was found. Something like this screenshot:

The value used above is:
export_rule1._found ? throwException("The volume is associated with an export policy that has active rules. Use Modify NFS Volume workflow to fix that before attempting to destroy the volume") : "No problem"
When the workflow is executed on a volume associated with an export policy with rules, then it errors out as shown below. Unfortunately, the error message, shows the complete expression string also.

When executed on a volume which is associated with an export policy with no rules, it succeeds.

The same can also be tried in the Advanced tab->Conditional Execution -> If the following expression is true.
If there is an easier way to achieve this case, then we will get back again.
Thanks,
Shailaja