make metadata not show in the manifest if nil

[kirkkwang]

According to pres 3 specs,

The value of the metadata property must be an array of JSON objects...

At the moment if there are no metadata, the metadata field is set to null which will thrown an error on the validator.

• add guard for metadata so it will not appear if nil

[kirkkwang]

Can do!

[kirkkwang]

accidentally change something because i misread it, fix here: https://github.com/samvera/iiif_manifest/pull/75