[#4540] fix(core): Support alter tag with null comment

[jerryshao]

What changes were proposed in this pull request?

Add NULL check for tag comment

Why are the changes needed?

Fix: #4540

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Add IT.

[yuqi1129]

Could you also check if this problem exists in other entities, I noticed that there is a similar operation in metalake
https://github.com/apache/gravitino/blob/6acf9c92924a20924f8778e98ca3c3adea446fe6/core/src/main/java/org/apache/gravitino/storage/relational/mapper/MetalakeMetaMapper.java#L137C1-L138C1

[jerryshao]

Could you also check if this problem exists in other entities, I noticed that there is a similar operation in metalake https://github.com/apache/gravitino/blob/6acf9c92924a20924f8778e98ca3c3adea446fe6/core/src/main/java/org/apache/gravitino/storage/relational/mapper/MetalakeMetaMapper.java#L137C1-L138C1

I guess so, but I don't have time to test all of them. I can check it in a separate PR.

[yuqi1129]

Could you also check if this problem exists in other entities, I noticed that there is a similar operation in metalake 6acf9c9/core/src/main/java/org/apache/gravitino/storage/relational/mapper/MetalakeMetaMapper.java#L137C1-L138C1

I guess so, but I don't have time to test all of them. I can check it in a separate PR.

OK

[yuqi1129]

I'm okay with the current changes.