If you never want a '' in that column in your database, perhaps you can use something (hideous) like:

INSERT INTO xyz.table (id,num, name,descr)
select id, num, name, case when descr='' then null else descr end
from (select ? as id, ? as num, ? as name, ? as descr) A
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and then use '' instead of undef to insert a null into the descr column...