Drop null `@language` values when expanding.
Drop null `@language` values when expanding.

- There's an inconsistency between the syntax spec and the API
  spec; the API spec throws an error if null is used for a
  `@language` value on Step 7.4.7 of the Expansion Algorithm,
  but the syntax spec allows it. When used, it indicates the
  value has no language -- which is the same as if `@language`
  were omitted, so it's treated the same way in this patch.

digitalbazaar/php-json-ld