fix: [QueryBuilder] `select()` does not escape after `NULL`

[kenjis]

Description Follow-up #1169

Fixes bug that select() does not escape items after NULL.

Checklist:

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