AuthorizationRequestServiceImpl does not handle `error` case

[oliverrahner]

Describe the bug
When trying to call extractAuthorizationResponseFromQuery with a query string that does not actually contain a signature, the call fails.

Exception in thread "main" java.lang.IllegalArgumentException: Parameter specified as non-null is null: method com.dke.data.agrirouter.api.dto.registrationrequest.secured.AuthorizationResponse.setSignature, parameter <set-?>
	at com.dke.data.agrirouter.api.dto.registrationrequest.secured.AuthorizationResponse.setSignature(AuthorizationResponse.kt)
	at com.dke.data.agrirouter.impl.onboard.secured.AuthorizationRequestServiceImpl.extractAuthorizationResponseFromQuery(AuthorizationRequestServiceImpl.java:88)
	at com.myagrirouter.example.App.main(App.java:150)

To Reproduce

AuthorizationRequestService authorizationRequestService = new AuthorizationRequestServiceImpl(environment);
var authorizationResult = extractAuthorizationResponseFromQuery("state=b02b4093-61c3-4eaa-b10b-cbc14cac2dad&error=request_declined")

Expected behavior

authorizationResult.error.equals("request_declined")