ToJSON: serialize string_columns to JSON

If the column is undefined, then it should be an empty string according to your
API spec.

src/lib/Hydra/Component/ToJSON.pm

@@ -18,6 +18,10 @@ sub TO_JSON {
         $json{$column} = $self->get_column($column);
     }
 
+    foreach my $column (@{$hint->{string_columns}}) {
+      $json{$column} = $self->get_column($column) // "";
+    }
+
     foreach my $column (@{$hint->{boolean_columns}}) {
         $json{$column} = $self->get_column($column) ? JSON::true : JSON::false;
     }

src/lib/Hydra/Schema/Projects.pm

@@ -247,7 +247,7 @@ __PACKAGE__->many_to_many("usernames", "projectmembers", "username");
 # DO NOT MODIFY THIS OR ANYTHING ABOVE! md5sum:+4yWd9UjCyxxLZYDrVUAxA
 
 my %hint = (
-    columns => [
+    string_columns => [
         "name",
         "displayname",
         "description",

t/Controller/projects.t

@@ -25,11 +25,11 @@ my $projectinfo = request(GET '/project/tests',
 
 ok($projectinfo->is_success);
 is(decode_json($projectinfo->content), {
-    description => undef,
+    description => "",
     displayname => "",
     enabled => JSON::true,
     hidden => JSON::false,
-    homepage => undef,
+    homepage => "",
     jobsets => [],
     name => "tests",
     owner => "root"