[fix](regression) Stabilize external table guardrail cases

[Gabriel39]

What problem does this PR solve?

The external-table regression suite had two environment-dependent failures:

• The Paimon guardrail case attempted to create a table with an empty field name. Paimon rejects empty and whitespace-only field names while constructing the table schema, before Doris can scan it.
• The Iceberg Variant case required more than one scanner to report rows. File scanner tasks pull ranges dynamically, so minimum scanner concurrency does not guarantee that every scheduled scanner consumes rows from a small multi-file fixture.

What is changed?

• Remove the unsupported empty-field Paimon fixture and its expected results. Delimiter-rich field names remain covered end to end, while empty-segment framing remains covered by the codec unit test.
• Keep the parallel Iceberg Variant correctness query, but remove the assertion on nondeterministic per-scanner range assignment.

Validation

• test_paimon_jni_reader_guardrails: passed
• test_iceberg_variant_read: passed twice consecutively

Release note

None. These are test-only corrections.

[Gabriel39]

run buildall

[hello-stephen]

Thank you for your contribution to Apache Doris.
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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?

[Gabriel39]

/review

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