Fallback com_users new_usertype to guest_usergroup or public and not hardcode group id's

[zero-24]

Pull Request for an issue raised by @bembelimen

Summary of Changes

Make sure the new_usertype is not hardcoded and in case it does not exists falls back to the guest_usergroup or the static id 1 / public.

Testing Instructions

Clear the default value for new_usertype and check that guest_usergroup or 1 is used.

Expected result

guest_usergroup or 1 is used when new_usertype is not configured

Actual result

fallback is hardcoded

Documentation Changes Required

none

cc @bembelimen @SniperSister @HLeithner

[Quy]

I have tested this item ✅ successfully on 3800d0b

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[HLeithner]

We need second test here.

[alikon]

I have tested this item ✅ successfully on 3800d0b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/24363.}

[ghost]

Status "Ready To Commit".

[HLeithner]

thx

[zero-24]

Thanks. 👍