Fix silent failures in checkExistingCategory

[giovanny07]

Summary

• Target entity missing from visible entities list: When ancestors_cache is
  empty (root entity or unpopulated cache after a DB migration/entity move), the
  target entity itself was never added to the ancestors array. Categories belonging
  directly to the target entity were silently reported as non-existent, causing them
  to be reset to empty on transfer.
• Unnecessary queries and loose-comparison pitfalls for tickets with no category:
  When itilcategories_id = 0, the function queried glpi_itilcategories WHERE id = 0
  (no results), leaving variables as empty strings. PHP loose comparison '' == 0
  evaluates to true, which could produce incorrect results depending on the target
  entity.
• No guard for deleted categories: If the category row no longer exists in
  glpi_itilcategories, the variables stayed as empty strings and the function relied
  on loose comparisons to reach a result instead of returning false immediately.
• Empty-string defaults and loose == throughout: The original code used '' as
  the default for entity IDs and booleans, then compared them with == and default
  (loose) in_array. PHP type coercion ('' == 0 → true, in_array('', [0]) →
  true) made the results unpredictable in edge cases.

Changes

src/Ticket.php — checkExistingCategory():

1. Always add $targetEntity to the visible entities list, regardless of whether
   ancestors_cache is populated.
2. Early return false when the ticket has no category (itilcategories_id = 0).
3. Early return false when the category does not exist in glpi_itilcategories.
4. Cast all IDs to int and flags to bool; use strict === and in_array(..., true).
5. Simplified visibility logic: a category that belongs directly to the target entity
   is always visible; otherwise it must be recursive and belong to an ancestor.

Test plan

• Transfer a ticket whose category belongs directly to the target entity (non-recursive)
  — category must be preserved
• Transfer a ticket whose category belongs to an ancestor of the target entity and is
  recursive — category must be preserved
• Transfer a ticket whose category belongs to an ancestor but is not recursive
  — category must be reset
• Transfer a ticket with no category (itilcategories_id = 0) — no errors, category
  stays empty
• Transfer a ticket whose category was deleted from glpi_itilcategories — category
  must be reset without errors
• Transfer to the root entity (entity 0) — verify correct behavior in all cases above