Is there a disk space impact when choosing NULL vs empty array {} for the default of an ARRAY[] column?
Why don't you check by yourself?

1 - Create 2 tables:

CREATE TABLE TABLE1 (column1 int not null, column2 int[] default {});
CREATE TABLE TABLE2 (column1 int not null, column2 int[]);


2  - make 1 million inserts for each table with:

insert into TABLE1(colulmn1) SELECT generate_series(1,1000000);
insert into TABLE2(colulmn1) SELECT generate_series(1,1000000);


3  - check table disk usage using this query :
(found in https://wiki.postgresql.org/wiki/Disk_Usage )

SELECT *, pg_size_pretty(total_bytes) AS total
    , pg_size_pretty(index_bytes) AS INDEX
    , pg_size_pretty(toast_bytes) AS toast
    , pg_size_pretty(table_bytes) AS TABLE
  FROM (
  SELECT *, total_bytes-index_bytes-COALESCE(toast_bytes,0) AS table_bytes FROM (
      SELECT c.oid,nspname AS table_schema, relname AS TABLE_NAME
              , c.reltuples AS row_estimate
              , pg_total_relation_size(c.oid) AS total_bytes
              , pg_indexes_size(c.oid) AS index_bytes
              , pg_total_relation_size(reltoastrelid) AS toast_bytes
          FROM pg_class c
          LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
          WHERE relkind = 'r' and relname in ('TABLE1','TABLE2')
  ) a
) a;


4 - whenever you are convinced just drop the tables:

DROP TABLE TABLE1;
DROP TABLE TABLE2;