When FlinkSQL writes ES, it will not write and update the null value field

Using Flink SQL to consume Kafka to write ES, sometimes some fields do not exist, and those that do not exist do not want to write ES, how to deal with this situation?

For example: the source data has 3 fields, a, b, c
insert into table2
select
a,b,c
from table1
When b=null, only hope to write a and c
When c=null, only hope to write a and b