Prior to this commit, the existing script does not work:
```
swh=> rollback; begin; update revision set extra_headers = ARRAY[][] where extra_headers is null;
ROLLBACK
BEGIN
ERROR: syntax error at or near "["
LINE 1: update revision set extra_headers = ARRAY[][] where extra_he...
```
With the following diff's content, on staging:
```
swh=> rollback; begin; update revision set extra_headers = ARRAY[]::bytea[][] where extra_headers is null;
ROLLBACK
BEGIN
UPDATE 865732
swh=> select extra_headers from revision where extra_headers = ARRAY[]::bytea[][] limit 10;
extra_headers
---------------
{}
...
```
I guess it's kinda related to T2524 ;)