Added default fallback for target when step is hidden/destroyed

[IWMTom]

Fixes #1131

If a step is added without specifically specifying an element to attach to, there was a default to fallback to document.body.
When a step was hidden/destroyed, this.target was checked which in the case of a non-attached step, this.target was null.

• Added a fallback to document.body if this.target is falsey when hiding/destroying a step
• Removed condition of this.target being truthy for calls to _updateStepTargetOnHide()
• Added unit tests to cover this scenario. Validated that the test failed prior to the fix, and that the test now passes.

[RobbieTheWagner]

Thanks for the PR!