bake: support null arg and label value

[crazy-max]

follow-up #1362 (comment)

variable "GO_VERSION" {
  default = ""
}
target "default" {
  args = {
    GO_VERSION = GO_VERSION
  }
}

ARG GO_VERSION="1.18"
FROM golang:${GO_VERSION}

$ docker buildx bake --print
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "GO_VERSION": ""
      }
    }
  }
}

In this example, GO_VERSION arg will always be sent in the request and overwrites the one in the Dockerfile. Therefore build would fail as GO_VERSION will be empty.

With this PR, null type is now handled and will set the arg only if set:

variable "GO_VERSION" {
  default = null
}
target "default" {
  args = {
    GO_VERSION = GO_VERSION
  }
}

$ docker buildx bake --print
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile"
    }
  }
}

$ GO_VERSION=1.19 docker buildx bake --print
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "GO_VERSION": "1.19"
      }
    }
  }
}

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[crazy-max]

Including a null value in a string template is not yet supported:

variable "GO_VERSION" {
  default = null
}
target "default" {
  args = {
    GO_VERSION = "${GO_VERSION}-alpine"
  }
}

This will lead to:

Invalid template interpolation value; The expression result is null. Cannot include a null value in a string template.

We could convert to the expected type.