fix(user): Can change display name

[Pytal]

• Fix [Bug]: Adding email addresses is disabled for users without one set already #38053

Summary

Regression from #37596

Fallback to true if

server/config/config.sample.php

Lines 242 to 246 in f8b4e00

	/**
	* ``true`` allows users to change their display names (on their Personal
	* pages), and ``false`` prevents them from changing their display names.
	*/
	'allow_user_to_change_display_name' => true,

is not set in config.php

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[Pytal]

Backports are not needed here @solracsf as #37596 was merged for 27 and master is still targeting 27 :)