Minor: Remove unneccessary vec! in SortMergeJoinStream initialization (apache#18430)

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Remove a unnecessary vec allocation in SortMergeJoinStream
initialization

## Rationale for this change

Remove a unnecessary vec allocation in SortMergeJoinStream
initialization

## What changes are included in this PR?

Remove a unnecessary vec allocation in SortMergeJoinStream
initialization

## Are these changes tested?

Covered by existing

## Are there any user-facing changes?

No

Author: mapleFU

datafusion/physical-plan/src/joins/sort_merge_join/stream.rs

@@ -604,7 +604,7 @@ impl Stream for SortMergeJoinStream {
                                             // Append filtered batch to the output buffer
                                             self.output = concat_batches(
                                                 &self.schema(),
-                                                vec![&self.output, &out_filtered_batch],
+                                                [&self.output, &out_filtered_batch],
                                             )?;
 
                                             // Send to output if the output buffer surpassed the `batch_size`