Is there a standard 'null' mime-type

Question

I am using a single JSON response format to cover a number of possible responses.

The response contains a field that, optionally, contains a link to an external reso

Answer 1

The IANA reserves "example" for, well, example purposes. But since "example" shall never be used as a real mime type, example/example could serve as an (internally used) compatible "null" MIME type.

https://www.iana.org/assignments/media-types/media-types.xhtml#examples

example

Note: The 'example' media type is used for examples. Any subtype following the media type syntax may be used in those examples. No subtype can be registered with IANA. For more information see[RFC4735].

Note: The occurrence of an 'example' media type as a media type outside of examples, e.g. in a Content-Type header, is an error and should be reported to the implementor.[RFC2045][RFC2046] specifies that Content Types, Content Subtypes, Character Sets, Access Types, and conversion values for MIME mail will be assigned and listed by the IANA.