[BugFix] Fix set the gtid of null rowset when publish (backport #55024)

[mergify]

Why I'm doing:

rowset in TxnManager can be null if it just prepare but not commit, so should check null before rowset->rowset_meta()->set_gtid(publish_version_req.gtid)

What I'm doing:

Fixes https://github.com/StarRocks/StarRocksTest/issues/9051

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr