[FrameworkBundle][Security] Replaced void return type with null for consistency
merged branch jakzal/dockblock-void-null-fix (PR #9026)

This PR was squashed before being merged into the 2.2 branch (closes #9026).

Discussion
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[FrameworkBundle][Security] Replaced void return type with null for consistency

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

We're using `null` everywhere else.

Commits
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e4100ac [FrameworkBundle][Security] Replaced void return type with null for consistency