fix: add missing validation for group name to `shield:user addgroup`/`removegroup`

[kenjis]

Description See https://github.com/codeigniter4/shield/pull/1164#discussion_r1722662886

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [x] Conforms to style guide