[Debug] simplify code path to remove potential blank pages

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8703
| License       | MIT
| Doc PR        | none

This should reduce the probability of hitting #8703 by removing an ob_start() from the error page output code path at bootstrap.
I would use ``null === $this->handler`` instead. It would be more consistent with the codebase