[fix][broker] Fix executeWithRetry result is null

[coderzc]

Motivation

In line 302 of MetadataCacheImpl#executeWithRetry,
the result of the future is set to a null value, which will lead to getting the result of readModifyUpdate and readModifyUpdateOrCreate will get a null value.
Fortunately, we haven't used this value for the time being.

pulsar/pulsar-metadata/src/main/java/org/apache/pulsar/metadata/cache/impl/MetadataCacheImpl.java

Lines 295 to 313 in 774c50b

	private CompletableFuture<T> executeWithRetry(Supplier<CompletableFuture<T>> op, String key) {
	CompletableFuture<T> result = new CompletableFuture<>();
	op.get().thenAccept(r -> result.complete(r)).exceptionally((ex) -> {
	if (ex.getCause() instanceof BadVersionException) {
	// if resource is updated by other than metadata-cache then metadata-cache will get bad-version
	// exception. so, try to invalidate the cache and try one more time.
	objCache.synchronous().invalidate(key);
	op.get().thenAccept((c) -> result.complete(null)).exceptionally((ex1) -> {
	result.completeExceptionally(ex1.getCause());
	return null;
	});
	return null;
	}
	result.completeExceptionally(ex.getCause());
	return null;
	});
	return result;
	}
	}

Modifications

Fix executeWithRetry result is null and modify the test to cover it.

Verifying this change

• Make sure that the change passes the CI checks.

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This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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[coderzc]

@rdhabalia PTAL