Correctly use initializeStats in DataSkippingStatsTracker

Add missing call to `initializeStats` inside of `DataSkippingStatsTracker`.

Existing UTs.

## Followup
Backport this to branch-1.2, branch-2.0 in Delta Lake, too.

GitOrigin-RevId: b4e7c08d14d0e8ba6b9c6c7a12cf2e63990e0ad9

core/src/main/scala/org/apache/spark/sql/delta/stats/DataSkippingStatsTracker.scala

@@ -131,14 +131,17 @@ class DeltaTaskStatisticsTracker(
 
   override def newPartition(partitionValues: InternalRow): Unit = { }
 
+  protected def initializeAggBuf(buffer: SpecificInternalRow): InternalRow =
+    initializeStats.target(buffer).apply(EmptyRow)
+
   override def newFile(newFilePath: String): Unit = {
     submittedFiles.getOrElseUpdate(newFilePath, {
       // `buffer` is a row that will start off by holding the initial values for the agg expressions
       // (see the initializeStats: Projection), will then be updated in place every time a new row
       // is processed (see updateStats: Projection), and will finally serve as an input for
       // computing the per-file result of statsColExpr (see getStats: Projection)
       val buffer = new SpecificInternalRow(aggBufferAttrs.map(_.dataType))
-      buffer
+      initializeAggBuf(buffer)
     })
   }