[WebBundle] Fix error in finalize step

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | no]
| Fixed tickets | None
| License       | MIT

If you have a checkout process consisting of only one step there is no previous step.  Hence the view gives an error.   A simple test is introduced to mitigate this problem