Make link 'value' optional

[tomhrr]

RFC 7483 has the following:

The JSON name/values of "rel", "href", "hreflang", "title", "media",
and "type" correspond to values found in Section 5 of [RFC5988].  The
"value" JSON value is the context URI as described by [RFC5988].  The
"href" JSON value MUST be specified.  All other JSON values are
OPTIONAL.

The validator treats "value" as mandatory, but it should be treated as optional.