Nullable types must be explicit

Looks good to me.

This of course also affects 3.x.

I still get a lot of depecations after applying the changes in the code:

PHP Deprecated:  field_group_form_process(): Implicitly marking parameter $form_state as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 426

Deprecated: field_group_form_process(): Implicitly marking parameter $form_state as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 426
PHP Deprecated:  field_group_field_group_form_process_build_alter(): Implicitly marking parameter $form_state as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 493

Deprecated: field_group_field_group_form_process_build_alter(): Implicitly marking parameter $form_state as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 493
PHP Deprecated:  field_group_fields_nest(): Implicitly marking parameter $vars as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 637

Deprecated: field_group_fields_nest(): Implicitly marking parameter $vars as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 637
PHP Deprecated:  field_group_field_layout_fields_nest(): Implicitly marking parameter $vars as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 730

Deprecated: field_group_field_layout_fields_nest(): Implicitly marking parameter $vars as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 730
PHP Deprecated:  field_group_group_save(): Implicitly marking parameter $display as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 877

Deprecated: field_group_group_save(): Implicitly marking parameter $display as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/www/modules/contrib/field_group/field_group.module on line 877

Updated MR with additional instances. Ready for review again.