no data field when there is no data

lib/Dancer2/Plugin/JsonApi/Registry/Schema.pm

@@ -59,7 +59,10 @@ sub serialize ( $self, $data, $extra_data = {} ) {
 
     my @included;
 
-    $serial->{data} = $self->serialize_data( $data, $extra_data, \@included );
+    if ( defined $data ) {
+        $serial->{data} =
+          $self->serialize_data( $data, $extra_data, \@included );
+    }
 
     $serial->{links} = gen_links( $self->top_level_links, $data, $extra_data )
       if $self->top_level_links;

t/registry-schema.t

@@ -7,10 +7,10 @@ use experimental qw/ signatures /;
 my $type =
   Dancer2::Plugin::JsonApi::Registry::Schema->new( 'type' => 'thing' );
 
-like $type->serialize( { attr1 => 'a', id => '123' },
-    { foo => 1 } ) => {
+like $type->serialize( { attr1 => 'a', id => '123' }, { foo => 1 } ) => {
     jsonapi => { version => '1.0' },
-    data    => { type    => 'thing', id => '123' } };
+    data    => { type    => 'thing', id => '123' }
+};
 
 is( Dancer2::Plugin::JsonApi::Registry::Schema->new(
         'type' => 'thing',
@@ -24,7 +24,8 @@ my $serialized = schema_serialize(
         id     => sub ($data) { $data->{x} . $data->{y} },
         links  => { self => '/some/url' },
     },
-    { x => '1', y => '2' } );
+    { x => '1', y => '2' }
+);
 
 is( $serialized->{data}{id} => '12',
     'custom id, function'
@@ -47,21 +48,18 @@ like(
             }
         }
       )->serialize( {}, { bar => 'yup' } ),
-    { meta => { foo => 1, bar => 'yup' } } );
-
+    { meta => { foo => 1, bar => 'yup' } }
+);
 
 subtest 'attributes' => sub {
     my $serialized =
-    Dancer2::Plugin::JsonApi::Registry::Schema->new(
-        type => 'thing',
-    )->serialize( { id => 1, foo => 'bar'});
+      Dancer2::Plugin::JsonApi::Registry::Schema->new( type => 'thing', )
+      ->serialize( { id => 1, foo => 'bar' } );
 
     is $serialized->{data} => {
         type       => 'thing',
         id         => 1,
-        attributes => {
-            foo => 'bar',
-        }
+        attributes => { foo => 'bar', }
     };
 
 };
@@ -86,7 +84,16 @@ subtest 'allowed_attributes', sub {
     is $serialized->{data} => {
         type       => 'thing',
         id         => 1,
-        attributes => { foo => 2, } };
+        attributes => { foo => 2, }
+    };
+};
+
+subtest 'empty data', sub {
+    my $serialized =
+      Dancer2::Plugin::JsonApi::Registry::Schema->new( type => 'thing' )
+      ->serialize(undef);
+
+    ok( !$serialized->{data}, "there is no data" );
 };
 
 done_testing();