Partial update on an postgres upsert violates constraint

Hi,
Basically I wanted to do a partial update inside pg (9.5), but it seems
that a partial update fails when not all of constraint is fulfilled (such
as the not null constraint)

Below are the sql queries I used,

CREATE TABLE jobs (
id integer PRIMARY KEY,
employee_name TEXT NOT NULL,
address TEXT NOT NULL,
phone_number TEXT);
CREATE OR REPLACE FUNCTION upsert_job(job JSONB)
RETURNS VOID AS $$BEGININSERT INTO jobs AS origin VALUES(
(job->>'id')::INTEGER,
job->>'employee_name'::TEXT,
job->>'address'::TEXT,
job->>'phone_number'::TEXT) ON CONFLICT (id) DO UPDATE SET
employee_name = COALESCE(EXCLUDED.employee_name, origin.employee_name),
address = COALESCE(EXCLUDED.address, origin.address),
phone_number = COALESCE(EXCLUDED.phone_number,
origin.phone_number);END;$$ LANGUAGE PLPGSQL SECURITY DEFINER;

--Full insert (OK)SELECT upsert_job('{"id" : 1, "employee_name" :
"AAA", "address" : "City, x street no.y", "phone_number" :
"123456789"}'::jsonb);
--Partial update that fulfills constraint (Ok)SELECT upsert_job('{"id"
: 1, "employee_name" : "BBB", "address" : "City, x street
no.y"}'::jsonb);
--Partial update that doesn't fulfill constraint (FAILS)SELECT
upsert_job('{"id" : 1, "phone_number" : "12345"}'::jsonb);
--ERROR: null value in column "employee_name" violates not-null
constraint--DETAIL: Failing row contains (1, null, null, 12345).

I also tried explicitly stating the columns that I wanted to insert, and it
also fails. How do I go around doing this ?

Thank you