fix(datafusion): return single row with count 0 for empty inserts

[u70b3]

Which issue does this PR close?

• Closes DataFusion empty INSERT returns empty batch and creates unnecessary snapshot #2713.

What changes are included in this PR?

Fix empty inserts in the DataFusion integration.

When an INSERT produces no data files (e.g. INSERT INTO ... SELECT ... WHERE false), IcebergCommitExec previously returned an empty RecordBatch. DataFusion expects a single-row count result for DML statements, so this PR changes the empty-data path to return a batch with count 0.

for empty inserts, the code returns early when there are no data files, before it ever starts a transaction, so it will not create snapshot.

Are these changes tested?

Yes. Added tests verifying that:

• Empty inserts return a single-row UInt64 count batch with value 0.
• Existing insert tests continue to pass.

Local verification:

• cargo test -p iceberg-datafusion
• cargo clippy -p iceberg-datafusion --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[huan233usc]

Code looks good — returning one row with count = 0 is the right fix, and it matches
what the normal (non-empty) path already does. Tests are fine too.

One small thing about the description: it says this PR also stops empty inserts from
creating a snapshot. But that part already works on main — the code returns early
when there are no data files, before it ever starts a transaction, so an empty insert
never created a snapshot in the first place. The only real change here is what the
batch looks like (empty → one row with count 0). The "no snapshot" checks in the new
tests are still nice to have as guards, they just aren't testing anything new.

Could you drop that "skips creating a snapshot" line from the description? Just so it
matches what the PR actually does. Otherwise good to go.

[CTTY]

Do you think we can migrate these tests to sqlogictest? The plan is to migrate off integration tests completely and use sqlogictest for logic like this

[u70b3]

yes, thanks for comment, already migrate some of the necessary testcase to sqlogictest