[SPARK-31159][SQL][FOLLOWUP] Move checking of the rebaseDateTime flag out of the loop in VectorizedColumnReader

### What changes were proposed in this pull request?
In the PR, I propose to refactor reading of timestamps of the `TIMESTAMP_MILLIS` logical type from Parquet files in `VectorizedColumnReader`, and move checking of the `rebaseDateTime` flag out of the internal loop.

### Why are the changes needed?
To avoid any additional overhead of the checking the SQL config `spark.sql.legacy.parquet.rebaseDateTime.enabled` introduced by the PR #27915.

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
By running the test suite `ParquetIOSuite`.

Closes #27973 from MaxGekk/rebase-parquet-datetime-followup.

Authored-by: Maxim Gekk <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: MaxGekk

sql/core/src/main/java/org/apache/spark/sql/execution/datasources/parquet/VectorizedColumnReader.java

@@ -462,15 +462,22 @@ private void readLongBatch(int rowId, int num, WritableColumnVector column) thro
           num, column, rowId, maxDefLevel, (VectorizedValuesReader) dataColumn);
       }
     } else if (originalType == OriginalType.TIMESTAMP_MILLIS) {
-      for (int i = 0; i < num; i++) {
-        if (defColumn.readInteger() == maxDefLevel) {
-          long micros = DateTimeUtils.millisToMicros(dataColumn.readLong());
-          if (rebaseDateTime) {
-            micros = DateTimeUtils.rebaseJulianToGregorianMicros(micros);
+      if (rebaseDateTime) {
+        for (int i = 0; i < num; i++) {
+          if (defColumn.readInteger() == maxDefLevel) {
+            long micros = DateTimeUtils.millisToMicros(dataColumn.readLong());
+            column.putLong(rowId + i, DateTimeUtils.rebaseJulianToGregorianMicros(micros));
+          } else {
+            column.putNull(rowId + i);
+          }
+        }
+      } else {
+        for (int i = 0; i < num; i++) {
+          if (defColumn.readInteger() == maxDefLevel) {
+            column.putLong(rowId + i, DateTimeUtils.millisToMicros(dataColumn.readLong()));
+          } else {
+            column.putNull(rowId + i);
           }
-          column.putLong(rowId + i, micros);
-        } else {
-          column.putNull(rowId + i);
         }
       }
     } else {