Give a default value for MediaMetadata constructor

[ChunMinChang]

By the definition of the optional argument:

If the type of an argument is a dictionary type or a union type that has a dictionary type as one of its flattened member types, and that dictionary type and its ancestors have no required members, and the argument is either the final argument or is followed only by optional arguments, then the argument must be specified as optional and have a default value provided.

Dictionary argument without any required fields need to have a default value. Hence

Constructor(optional MediaMetadataInit init)

should be replaced by

Constructor(optional MediaMetadataInit init = {})

Given that the extended attributes for Constructor is replaced by the new constructor() syntax(#235), the Constructor should be moved in the MediaMetadata interface and replaced by

constructor(optional MediaMetadataInit init = {})

[ChunMinChang]

Fixed in #236