Bug 23049: (QA follow-up) Fix swagger spec for account line

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

api/v1/swagger/definitions/account_line.json

@@ -40,11 +40,17 @@
       "description": "Account line description"
     },
     "account_type": {
-      "type": "string",
+      "type": [
+          "string",
+          "null"
+      ],
       "description": "Account line type"
     },
     "debit_type": {
-      "type": "string",
+      "type": [
+          "string",
+          "null"
+      ],
       "description": "Account line debit type"
     },
     "payment_type": {