[BugFix] fix iceberg read null partition bug

[Wenjun7J]

Why I'm doing:

What I'm doing:

Fixes #issue

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

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

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

[Java-Extensions Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 18 / 18 (100.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	com/starrocks/connector/iceberg/IcebergConnectorScanRangeSource.java	4	4	100.00%	[]
🔵	com/starrocks/connector/partitiontraits/IcebergPartitionTraits.java	10	10	100.00%	[]
🔵	com/starrocks/connector/PartitionUtil.java	4	4	100.00%	[]

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)