Bug 26271: Add null to the list of accepted account_line data types

The database schema for accountlines table allows the
manager_id/user_id column to be NULL. If request to
/api/v1/patrons/<patron_id>/account returns such an accountline where
it is NULL we get 500 error as response. Adding NULL to allowed data
types fixes this issue.

To test:
 1) Run prove t/db_dependent/api/v1/patrons_accounts.t and notice it
    doesn't fail

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

api/v1/swagger/definitions/account_line.json

@@ -84,7 +84,10 @@
       "description": "Internal note"
     },
     "user_id": {
-      "type": "integer",
+      "type": [
+        "integer",
+        "null"
+      ],
       "description": "Internal patron identifier for the staff member that introduced the account line"
     },
     "library_id": {