[schema change](fix) fix coredump of schema change (#13183)

When schema change and compaction is executing simutaneously, both
nullable and not nullable data can be read for the same column, need to
reset _nullmap for each Block when converting Block data, or else Column
case will be wrong.

be/src/vec/olap/olap_data_convertor.cpp

@@ -160,6 +160,7 @@ void OlapBlockDataConvertor::OlapColumnDataConvertorBase::set_source_column(
 void OlapBlockDataConvertor::OlapColumnDataConvertorBase::clear_source_column() {
     // just to reduce the source column's ref count to 1
     _typed_column.column = nullptr;
+    _nullmap = nullptr;
 }
 
 // This should be called only in SegmentWriter. If you want to access nullmap in Convertor,