I think @Kabirdas workaround from the same thread is the simplest Band-Aid to this problem.
he only mentions that text columns, other than 'date', will read null as "" but from what you're saying it sounds like null integer does the same. have you tried this with Nullable as false or with INT8?
its a bit outside the box, but you could also try with Number type as float and PostgreSQL type as FLOAT4, the db should implicitly cast integers to floats on insert statements. floats have 3 special values no other numeric type has, specifically NaN and what you might find is that when Retool tries to insert "" into a float column PostgreSQL casts it to NaN instead of throwing that error. as long as you aren't doing complex math or maybe lots of comparisons then I wouldn't think the performance hit would be too bad