[Bugfix] Fix iceberg partition last updated null

[eshishki]

Why I'm doing:

ERROR (connector-trigger-analyze-pool-0|365) [IcebergCatalog.getPartitionLastUpdatedTime():374] Failed to get last_updated_at for partition [_cdc_tsms_day=2025-04-22] of table [iceberg.land
ing_cdc_cbs_tj.credit_schedule_payments] under snapshot [2358892543328080287]
java.lang.NullPointerException: Cannot invoke "java.lang.Long.longValue()" because the return value of "org.apache.iceberg.StructLike.get(int, java.lang.Class)" is null
        at com.starrocks.connector.iceberg.IcebergCatalog.getPartitionLastUpdatedTime(IcebergCatalog.java:372) ~[fe-core-main.jar:?]
        at com.starrocks.connector.iceberg.IcebergCatalog.getPartitions(IcebergCatalog.java:350) ~[fe-core-main.jar:?]

What I'm doing:

Check for NPE

@LiShuMing related to #65969

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 pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.0
  • 3.5
  • 3.4
  • 3.3

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[Java-Extensions Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)