Exporting a feature with field groups sometimes leads to $field_group->data being set to an empty string

Exporting a feature with field groups sometimes leads to $field_group->data being set to an empty string.

When this empty string is tested by field_group_unpack() $packed_group->data is set, but the reference to $group->label raises a warning.

function field_group_unpack($packed_group) {
if (!isset($packed_group->data)) {
return $packed_group;
}

// Extract unserialized data.
$group = clone $packed_group;
...
$group->label = $data['label'];

I've attached a patch that address this by testing for empty() instead of !isset().

Patch file field_group_patch-2189777-1.patch is for version 1.1
Patch file field_group_patch-2189777-2.patch is for version 1.3