## 1. Create API default

- [x] 1.1 In `EmployeeController::store()`, change the contract position fallback condition from `!\request()->has('contract_position')` to `blank($employeeData['contract_position'] ?? null)`, keeping the value `$model->positions()->first()->name ?? null`

## 2. Excel import default

- [x] 2.1 In `EmployeeController::import()`, read the "Vị trí HĐ" cell with `trim(... ?? '')`
- [x] 2.2 Set `contract_position` in the `$employee` data to the trimmed cell value when it is non-empty, otherwise to the resolved `$positionName`

## 3. Verification

- [x] 3.1 Create API: verify `contract_position` defaults to the first position's name when omitted, `null`, `""` or whitespace, and is kept when a value is sent
- [x] 3.2 Import: verify a blank "Vị trí HĐ" with a valid "Vị trí" gives that position's name, a blank "Vị trí HĐ" with an empty or unknown "Vị trí" gives the default position's name, and a filled "Vị trí HĐ" is kept (trimmed) (verified manually)
- [x] 3.3 Update API: verify that sending `contract_position` as `null` or `""` still stores it as empty, with no fallback
- [x] 3.4 Generate a contract for a newly created or imported employee with blank input and confirm the position field is rendered (verified manually)