What does this PR do?

Fixes #8785

Problem

When a client explicitly sends name: null in the request body to any of the user creation endpoints (POST /v1/users, /v1/users/bcrypt, /v1/users/md5, etc.), the API returns a vague server error instead of handling it gracefully.

Solution

The name parameter now uses the Nullable validator wrapper (consistent with email and phone parameters that already handle null values):

1. Changed the param validator from new Text(128) to new Nullable(new Text(128))
2. Changed the action function parameter type from string $name to ?string $name
3. Used null coalescing operator ($name ?? '') when passing to createUser() function

Affected Endpoints

• POST /v1/users
• POST /v1/users/bcrypt
• POST /v1/users/md5
• POST /v1/users/argon2
• POST /v1/users/sha
• POST /v1/users/phpass
• POST /v1/users/scrypt
• POST /v1/users/scrypt-modified

Testing

• Sending name: null now creates user with empty name (same as not sending name at all)
• Sending name: "John" works as before
• Not sending name parameter works as before

Related Issue

Fixes #8785