Fix NPE in UserAccountManagerImpl.getUser(CharSequence)

Optional.of() called on nullable value causes NPE.
We should use Optional.ofNullable() instead, as the
argument can be null by design.

Fixes #5155

Signed-off-by: Chris Narkiewicz <hello@ezaquarii.com>

src/main/java/com/nextcloud/client/account/UserAccountManagerImpl.java

@@ -227,7 +227,7 @@ public class UserAccountManagerImpl implements UserAccountManager {
     public Optional<User> getUser(CharSequence accountName) {
         Account account = getAccountByName(accountName.toString());
         User user =  createUserFromAccount(account);
-        return Optional.of(user);
+        return Optional.ofNullable(user);
     }
 
     @Override