fix(users): persist cleared custom attribute hints

[ouchihao]

Clearing an existing custom attribute description or placeholder converts the empty string to undefined, so JSON omits the field and the update API retains the old value. Send the empty strings explicitly so these fields can be cleared.

Validation:

• Covers clearing each field and both together, updating nonempty text and creating an attribute without optional text. 3 regression cases fail before the fix; all 5 focused cases pass after it.
• Focused tests plus the 17 Makefile critical frontend suites: 232 tests passed.
• vue-tsc --noEmit, full frontend ESLint and git diff --check passed.

Validation uses component tests with mocked APIs; no live server testing.