my_text,my_date,my_int
'Some Text','1/1/18',3
'More Text,,2
'Enough','',1
CREATE TEMP TABLE my_test (my_text TEXT, my_date DATE, my_int INT);
\copy my_test FROM test.csv WITH CSV HEADER
ERROR: invalid input syntax for type date: "''"
CONTEXT: COPY my_test, line 4, column my_date: "''"
You'll note that it breaks on the last line, which is like yours, not the one before it.
I think there may be some other ways to specify the NULL value, but I'm not really sure about that part.
Cheers,
Ken