RPDE feed not serialized correctly for empty pages

[nathansalter]

Currently when serializing an RPDE feed with an empty page the items key is blank.

Example code:

$feedPage = RpdeBody::createFromModifiedId(
    'http://example.com',
    1585574114067820,
    'c8de7e8b-f035-4a1f-a588-4c788176a0e7',
    []
);
$serialized = RpdeBody::serialize($feedPage);

Expected response:

{
  "next": "\/api\/open-active\/individual-facility-uses?afterTimestamp=1585574114067820&afterId=c8de7e8b-f035-4a1f-a588-4c788176a0e7",
  "license": "https:\/\/creativecommons.org\/licenses\/by\/4.0\/",
  "items": []
}

Actual response:

{
  "next": "\/api\/open-active\/individual-facility-uses?afterTimestamp=1585574114067820&afterId=c8de7e8b-f035-4a1f-a588-4c788176a0e7",
  "license": "https:\/\/creativecommons.org\/licenses\/by\/4.0\/"
}

[nathansalter]

Fixed in #49