on-error event is emitted even when error is null

[chriscyang]

https://github.com/BinarCode/vue-form-wizard/blob/ff92af9fc2e877e41a381cf29e6bb14c8c605184/src/components/FormWizard.vue#L343-L346

There are several methods that call this.setValidationError(null) in order to reset the error message. However, setValidationError always emits on-error regardless of the value of error. It took me a while to figure out why my on-error callback was always being executed when I was going back to the previous step.

This is inconsistent with the behaviour described in the documentation which says that on-error is called when before-change is promised and is rejected with a message.