iceberg maintenance: read the returned row before treating EOF as empty

parquet-go can return a row alongside io.EOF; a one-row merged file then
lost its first_row_id and the manifest list would allocate a fresh range.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/worker/tasks/iceberg/row_lineage.go

@@ -372,12 +372,13 @@ func mergedFileFirstRowID(data []byte) (*int64, error) {
 		return nil, nil
 	}
 	rows := make([]parquet.Row, 1)
-	if _, err := reader.ReadRows(rows); err != nil {
-		if err == io.EOF {
-			return nil, nil
-		}
+	n, err := reader.ReadRows(rows)
+	if err != nil && err != io.EOF {
 		return nil, err
 	}
+	if n == 0 {
+		return nil, nil
+	}
 	for _, v := range rows[0] {
 		if v.Column() != leaf.ColumnIndex {
 			continue