chore: update openapi

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

apps/files_sharing/openapi.json

@@ -686,7 +686,8 @@
                         "type": "string"
                     },
                     "url": {
-                        "type": "string"
+                        "type": "string",
+                        "nullable": true
                     }
                 }
             },