[improve] Allow the content be empty when deserialize in MetadataCache

[zymap]

---

Motivation

When using admin to create topic, it will set the content to byte[0] then put it into the value. Sometimes we need to update the value then we will received the deserailization error.
So we need to check the value type to make it as optional empty when the value is null.

Fixes #xyz

Main Issue: #xyz

PIP: #xyz

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[lhotari]

LGTM

[horizonzy]

LGTM.

[lhotari]

instead of making the change above, Optional.of could be changed to Optional.ofNullable to achieve the same outcome.

[lhotari]

There would be less code duplication if this change would be handled in a way where obj is set to null when res.getValue().length == 0

[lhotari]

for example T obj = res.getValue().length > 0 ? serde.deserialize(path, res.getValue(), res.getStat()) : null;

[hangc0276]

BookieServiceInfo handled the case of res.getValue.length==0 in deserialize

pulsar/pulsar-metadata/src/main/java/org/apache/pulsar/metadata/bookkeeper/BookieServiceInfoSerde.java

Lines 72 to 74 in 1b43b9d

	if (bookieServiceInfo == null || bookieServiceInfo.length == 0) {
	return BookieServiceInfoUtils.buildLegacyBookieServiceInfo(bookieId);
	}

That's why the test TestAutoRecoveryAlongWithBookieServers#testAutoRecoveryAlongWithBookieServers failed.

Maybe we need to deserialize first and add the res.getValue.length==0 check in the exception section?

[zymap]

@hangc0276 Thank you! I realized I was wrong to do this fix here because the byte[0] may be a magic value for the SerDe implementation. It should be handled in the SerDe classes.

[hangc0276]

BookieServiceInfo handled the case of res.getValue.length==0 in deserialize

pulsar/pulsar-metadata/src/main/java/org/apache/pulsar/metadata/bookkeeper/BookieServiceInfoSerde.java

Lines 72 to 74 in 1b43b9d

	if (bookieServiceInfo == null || bookieServiceInfo.length == 0) {
	return BookieServiceInfoUtils.buildLegacyBookieServiceInfo(bookieId);
	}

That's why the test TestAutoRecoveryAlongWithBookieServers#testAutoRecoveryAlongWithBookieServers failed.

Maybe we need to deserialize first and add the res.getValue.length==0 check in the exception section?