Which issue does this PR close?

• Partially closes Implement partition_statistics API for more operators apache/datafusion#15873

Rationale for this change

EmptyExec previously returned "unknown" statistics for both global and partition-level queries, which impacts planner accuracy. This change provides exact-zero stats for EmptyExec, aligning with how other operators like AggregateExec and PlaceholderRowExec handle partition statistics.

What changes are included in this PR?

• Implemented partition_statistics(...) for EmptyExec:
  • Sets num_rows and total_byte_size to Precision::Exact(0)
  • Populates column_statistics with one ColumnStatistics::new_unknown() per schema field
• Added unit test empty_multi_partition_statistics:
  • Verifies default 1-partition behavior: global and partition 0 return zero stats, invalid partition errors
  • Verifies 2-partition behavior: partitions 0 and 1 return zero stats, invalid partition errors
• Added a TreeRender branch to DisplayAs to provide informative display of the operator:
  "EmptyExec: partitions=X, fields=Y"
• No other execution plans or tests were modified

Are these changes tested?

Yes.

• New test covers all relevant edge cases.
• The full test suite still passes without regressions.

Are there any user-facing changes?

No.
This change affects internal planner behavior only; no public APIs or outputs are changed.