mgr/dashboard: fix oauth2-service creation UI error

mgr/dashboard: fix oauth2-service creation UI error

While creating the service without providing the allowlist domain, the
UI fails with an error which is logged in the mgr log

```
Nov 05 04:11:56 ceph-node-00 ceph-mgr[1587]: [dashboard ERROR frontend.error] (https://192.168.100.100:8443/#/services/(modal:create)): Cannot read properties of null (reading 'split')
                                              TypeError: Cannot read properties of null (reading 'split')
                                                 at ServiceFormComponent.onSubmit (https://192.168.100.100:8443/src_bootstrap_ts.js:31997:74)
                                                 at ServiceFormComponent_Template_cd_form_button_panel_submitActionEvent_60_listener (https://192.168.100.100:8443/src_bootstrap_ts.js:34168:83)
                                                 at executeListenerWithErrorHandling (https://192.168.100.100:8443/node_modules_angular_core_fesm2022_core_mjs.js:26276:12)
                                                 at Object.wrapListenerIn_markDirtyAndPreventDefault [as next] (https://192.168.100.100:8443/node_modules_angular_core_fesm2022_core_mjs.js:26308:18)
                                                 at SafeSubscriber.__tryOrUnsub (https://192.168.100.100:8443/default-node_modules_rxjs__esm2015_internal_AsyncSubject_js-node_modules_rxjs__esm2015_intern-7c6e1a.js:960:10)
                                                 at SafeSubscriber.next (https://192.168.100.100:8443/default-node_modules_rxjs__esm2015_internal_AsyncSubject_js-node_modules_rxjs__esm2015_intern-7c6e1a.js:900:14)
                                                 at Subscriber._next (https://192.168.100.100:8443/default-node_modules_rxjs__esm2015_internal_AsyncSubject_js-node_modules_rxjs__esm2015_intern-7c6e1a.js:847:22)
                                                 at Subscriber.next (https://192.168.100.100:8443/default-node_modules_rxjs__esm2015_internal_AsyncSubject_js-node_modules_rxjs__esm2015_intern-7c6e1a.js:824:12)
                                                 at EventEmitter_.next (https://192.168.100.100:8443/default-node_modules_rxjs__esm2015_internal_AsyncSubject_js-node_modules_rxjs__esm2015_intern-7c6e1a.js:604:17)
                                                 at EventEmitter_.emit (https://192.168.100.100:8443/node_modules_angular_core_fesm2022_core_mjs.js:7069:13)
```

Fixes: https://tracker.ceph.com/issues/73717
Signed-off-by: Nizamudeen A <nia@redhat.com>