make the link type optional

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

without this I was not able to submit the form using link type ``auto`` but I am not sure how this feature is intended to work in detail.

/cc @dbu