[improvement](fe) Skip zero-length files in FileSplitter to avoid sending empty splits to BE

[kaka11chen]

Issue Number: close #xxx

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

Zero-length files (length=0) on external storage (HDFS/S3) can produce empty splits that are assigned to BE, wasting RPC and scheduling resources. FileSplitter.splitFile() previously created splits even for length=0 files (specifically for non-splittable/compressed files). These splits carry no data but still consume a scan range slot and backend scheduling overhead.

This does NOT affect:

• Paimon JNI splits (length=0 but data in Paimon split object)
• Hudi log-only splits (length=0 but data in delta logs)
• Iceberg splits (create their own split objects)
  These formats bypass FileSplitter and create splits directly.

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[Thearas]

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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Copilot]

Pull request overview

This PR prevents generation of empty scan splits for zero-length external files (e.g., HDFS/S3), reducing unnecessary BE scheduling/RPC overhead.

Changes:

• Add an early return in FileSplitter.splitFile() to skip length <= 0 files.
• Add a unit test covering splittable/non-splittable and null block-location cases for zero-length files, including ensuring the initial-split counter isn’t consumed.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
fe/fe-core/src/main/java/org/apache/doris/datasource/FileSplitter.java	Skips zero-length files to avoid creating empty splits.
fe/fe-core/src/test/java/org/apache/doris/datasource/FileSplitterTest.java	Adds coverage ensuring zero-length files produce no splits and don’t consume initial split quota.

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[Copilot]

The early return for zero-length files currently allocates a new mutable list on every call. Since this is a hot path when scanning many files, consider returning Collections.emptyList() (or ImmutableList.of()) to avoid unnecessary allocations and signal immutability.

Suggested change

	return Lists.newArrayList();
	return ImmutableList.of();

[kaka11chen]

run buildall