fix(note): fix type for owner param

To make the create method easier to use in conjunction
with the authentication framework, this commit changes the type of
the `owner` parameter from `User | undefined` to `User | null`.

Signed-off-by: David Mehren <git@herrmehren.de>

src/notes/notes.service.ts

@@ -88,8 +88,8 @@ export class NotesService {
    */
   async createNote(
     noteContent: string,
+    owner: User | null,
     alias?: string,
-    owner?: User,
   ): Promise<Note> {
     if (alias) {
       this.checkNoteIdOrAlias(alias);