Land document dates to statements on merge

[puerco]

When merging documents, any statements without any dates should inherit the document date. Currently, statements which don't have timestamps will be merged without dates, corrupting the VEX history.

Here is a sample merged doc:

{
  "id": "vex-8a489e826e47a7a9bc8c4a7b37a2d8033360f06c29f65433d17794a881e2d735",
  "format": "text/vex",
  "author": "",
  "role": "",
  "timestamp": "2022-12-28T13:31:27.77315253-06:00",
  "statements": [
    {
      "vulnerability": "CVE-1234-5678",
      "products": [
        "pkg:apk/wolfi/git@2.39.0-r0?distro=wolfi",
        "pkg:apk/wolfi/git-daemon@2.39.0-r0?distro=wolfi",
        "pkg:apk/wolfi/git-email@2.39.0-r0?distro=wolfi"
      ],
      "status": "not_affected"
    }
}