pg: Check revision.extra_headers is not null.

Fixes a regression in 038a219f,
as it made the converter expect a list.

When adding this column, we made it default to null instead of defaulting
to an empty array, so existing records were initialized will null.

This commit migrates these nulls to empty arrays, then adds a constraint
to enforce it in the future.

sql/upgrades/161.sql

+-- SWH DB schema upgrade
+-- from_version: 160
+-- to_version: 161
+-- description: Make revision.extra_headers not null
+
+-- latest schema version
+insert into dbversion(version, release, description)
+      values(161, now(), 'Work Still In Progress');
+
+
+update revision
+    set extra_headers = ARRAY[][]
+    where extra_headers is null;
+
+
+alter table revision
+    alter column extra_headers set not null;

swh/storage/sql/30-swh-schema.sql

@@ -17,7 +17,7 @@ comment on column dbversion.description is 'Release description';
 
 -- latest schema version
 insert into dbversion(version, release, description)
-      values(160, now(), 'Work In Progress');
+      values(161, now(), 'Work In Progress');
 
 -- a SHA1 checksum
 create domain sha1 as bytea check (length(value) = 20);
@@ -240,7 +240,7 @@ create table revision
   object_id             bigserial,
   date_neg_utc_offset   boolean,
   committer_date_neg_utc_offset boolean,
-  extra_headers         bytea[][]  -- extra headers (used in hash computation)
+  extra_headers         bytea[][] not null  -- extra headers (used in hash computation)
 );
 
 comment on table revision is 'A revision represents the state of a source code tree at a specific point in time';