editor: fix edition statement saving problem.

According to PO, the 'responsibilities' field from document 'edition
statement' should be optional in the editor form. This commit hides this
field (if it's empty) so, now, the user must choose to add this field
(and fill a value). Editing a document without 'responsibilities' is now
allowed by default.

* Closes rero#906

 # the title begins with a verb, imperative mode
component: summarize changes in 50 char or less

* Adds your new feature.
* Fixes an existing issue, may closes #issue.
* Improves and existing feature.

Co-Authored-by: Renaud Michotte <renaud.michotte@gmail.com>

rero_ils/modules/documents/jsonschemas/documents/document-v0.0.1.json

@@ -524,6 +524,9 @@
             "items": {
               "title": "Responsibility",
               "$ref": "#/definitions/language_script"
+            },
+            "form": {
+              "hide": true
             }
           }
         }