FIX select on ticket list for fields group and severity (issue #31619)

[thomas-Ngr]

FIX select group and severity search fields on ticket list

issue #31619 (indicates 20.0, but I encountered it also on develop)
see also PR #32120 , which is similar but for ticket type, and brings more details.

group and severity must be set when creating/updating a ticket, and have a default value.
However, on search fields, we don't want to set the default value.

This PR sets the default value only if $empty is not set.
These are the changes brought by this PR :

• when $empty is set (case search lists), the empty value is selected
• when $empty value is not set, the default value is selected. If there is no default value, an obviously wrong value ('Select group', 'Select severity') is used.

if no default value :