Add null check in DelegatingBlockReader#toString() and unit tests

[LSukie]

What changes are proposed in this PR?

This PR adds a null check in the DelegatingBlockReader#toString() method to handle cases where the underlying BlockReader is null. This prevents potential NullPointerException during the toString() call.

Additionally, unit tests were added to verify this new behavior.

Why are the changes needed?

The change ensures that the DelegatingBlockReader can gracefully handle cases where the underlying BlockReader is null, which improves robustness.

Does this PR introduce any user-facing changes?

No.

How was this patch tested?

Unit tests have been added and verified with the current testing framework.

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[LSukie]

alluxio-bot, check this please.

[alluxio-bot]

Automated checks report:

• PR title follows the conventions: PASS
• Commits associated with Github account: PASS

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