Federation extension: nullable last_successful_check (#560)

* Federation extension: nullable last_successful_check

when back-end was never observed to be available

* Update extensions/federation/README.md

Co-authored-by: Matthias Mohr <matthias@mohr.ws>

* PR #560 add changelog entry

---------

Co-authored-by: Matthias Mohr <matthias@mohr.ws>

Author: soxofaan

CHANGELOG.md

@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Clarified how the relation types `license`, `version-history` and `author` can be used to enrich the process metadata. [#531](https://github.com/Open-EO/openeo-api/issues/531)
 - Clarified the behaviour of `federation:backends` for `POST /validation`
 - Clarified the meaning of `expires` in batch job results
+- Clarified that `last_successful_check` (from Federation Extension) can be null.
 
 ## [1.2.0] - 2021-05-25
 

extensions/federation/README.md

@@ -61,8 +61,10 @@ schema:
           last_successful_check:
             type: string
             format: date-time
+            nullable: true
             description: >-
-              If the `status` is `offline`: The time at which the back-end was checked and available the last time.
+              If the `status` is `offline`: The time at which the back-end was checked and available the last time
+              or `null` when the back-end was never observed to be available.
               Otherwise, this is equal to the property `last_status_check`.
           experimental:
             type: boolean