Unmarshal empty lists, allow for adjustment-only build matrix

[pda]

Perhaps it should be possible to define empty dimension lists, and then specify values for those dimensions via adjustments.

This pull request is currently just a failing test just some failing tests some tests by @pda and a solution written by @DrJosh9000 🙏🏼

Currently, an empty list gets converted to null which gets rejected server-side as being an invalid value for a matrix dimension value.

e.g. currently a pipeline like this:

steps:

  - command: "echo {{matrix.color}} {{matrix.shape}}"
    label: "{{matrix.color}} {{matrix.shape}}"
    matrix:
      setup:
        color: []
        shape: []
      adjustments:
        - with: { color: "green", shape: "hexagon" }
        - with: { color: "blue", shape: "triangle" }

is rejected with this error:

fatal: Failed to upload and process pipeline: Pipeline upload rejected: Each item within a matrix must be either a string, boolean or integer

Because the setup is uploaded as "setup":{"color":null,"shape":null}.

[DrJosh9000]

Nice test additions 👍