sql/swh-schema.sql: add default values and indexes on person

sql/swh-schema.sql

@@ -231,10 +231,12 @@ create table directory_list_rev
 create table person
 (
   id     bigserial primary key,
-  name   text,
-  email  text
+  name   text not null default '',
+  email  text not null default ''
 );
 
+create unique index on person(name, email);
+
 create type revision_type as enum ('git', 'tar', 'dsc');
 
 -- A snapshot of a software project at a specific point in time.