fix(spanner): fix write replace used by dataflow template and import export (#2901)

Author: harshachinta

google-cloud-spanner/src/main/java/com/google/cloud/spanner/GrpcStruct.java

@@ -70,6 +70,7 @@ private Object writeReplace() {
     for (int i = 0; i < structFields.size(); i++) {
       Type.StructField field = structFields.get(i);
       String fieldName = field.getName();
+      ensureDecoded(i);
       Object value = rowData.get(i);
       Type fieldType = field.getType();
       switch (fieldType.getCode()) {