Compute correct full_name when route name absent

What does this MR do and why?

There is a number of groups whose associated routes.name is NULL. With the cached_route_lookups feature flag enabled, the #full_name of these groups computes to nil.

Concerns 14106 groups on production.

More context:

• !111199 (comment 1288574247)

How to set up and validate locally

UPDATE routes
SET name = NULL
WHERE source_type = 'Namespace'
AND source_id = 209;

Feature.enable(:cached_route_lookups)
=> true
Group.find(209).full_name
=> nil
Feature.disable(:cached_route_lookups)
=> true
Group.find(209).full_name
=> "Test Group"

MR acceptance checklist

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