Allow removing default headers

[jakearchibald]

Description

I think this is a regression, as it was added in vercel/serve-handler#45, and there appears to be tests for it.

If I try to launch serve with the following config:

{
  "headers": [
    {
      "source": "**/*.*",
      "headers": [
        {
          "key": "Content-Disposition",
          "value": null
        }
      ]
    }
  ]
}

It fails with:

ERROR  The configuration you provided is invalid:
must be string
{"type":"string"}

Library version

14.2.4

Node version

v22.18.0

[jakearchibald]

https://github.com/vercel/schemas/blob/f78f1110a2c2d06d8ecc05999a8cd3fccd06670b/deployment/config-static.js#L49 here's the issue. I'm not sure what the format is, but that needs to allow null.

[jakearchibald]

PR vercel/schemas#89