Added null check when printing the brand with --print-ast.

Bug: chromium:961507, chromium:961508

Original change's description:
> [class] implement private method declarations
>
> This patch implements the declarations of private methods, the access
> of private methods would be left to a future patch.
> When a private methods declaration is encountered, we now:
>
> - Create a brand symbol during class evaluation and store it in the
>   context.
> - Create the closures for the private methods
> - Load the brand from the context and store it in the instance in the
>   constructor.
>
> Design: https://docs.google.com/document/d/1T-Ql6HOIH2U_8YjWkwK2rTfywwb7b3Qe8d3jkz72KwA/edit#
>
> Bug: v8:8330
> Change-Id: I2d695cbdc8a7367ddc7620d627b318f779d36150
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1568708
> Commit-Queue: Joyee Cheung <joyee@igalia.com>
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61387}

Change-Id: I3bf465f70c27914c9ec19f3f59ae018b28c9a866
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605521
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61459}