Fix back-office category save payload normalization

[CySSoO]

Motivation

• Categories could fail to save from the back office because the controller form sometimes posts a nested category[...] payload while the assembler expected a flat array.
• An empty string for id_parent_category could be persisted as '' which may cause issues with DB constraints or relation checks.

Description

• Added an extractPayload helper to CategoryCommandAssembler and call it from assembleCreate and assembleUpdate to accept both nested category[...] payloads and flat arrays.
• Coerce an empty id_parent_category string to null in mergeDefaults so parent IDs are either integers or null.
• Keep existing default merging behavior and apply the normalization for both create and update command assembly.

Testing

• Ran php -l src/Application/Blog/CategoryCommandAssembler.php which reported no syntax errors (success).

---

Codex Task