allow null value in fragment handler
bug #10067 [HttpKernel] allow null value in fragment handler (kbond)

This PR was submitted for the 2.4-dev branch but it was merged into the 2.4 branch instead (closes #10067).

Discussion
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[HttpKernel] allow null value in fragment handler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10066
| License       | MIT
| Doc PR        | n/a

Commits
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534deef [HttpKernel] allow null value in fragment handler