fix: allow using function only to patch to and from the environment

[phisco]

From https://crossplane.slack.com/archives/CEG3T90A1/p1716529989392019:

Details

Hello, hopefully someone here can point me in the right direction.
I'm struggling a bit to understan pipeline context. My current assumption is that I should be able to run a function-patch-and-transform with a ToEnvironmentFieldPath patch to write a value to the context.
This is my setup:
$ crossplane version
Client Version: v1.16.0
Server Version: v1.15.2-up.1
XR from function-patch-and-transform multistep example
functions from function-patch-and-transform multistep example
composition from function-patch-and-transform multistep example with the addition below:
...

- step: patch-and-transform-to-env
  functionRef:
    name: function-patch-and-transform
  input:
    apiVersion: pt.fn.crossplane.io/v1beta1
    kind: Resources
    resources:
      - name: bucketACL 
        patches:
          - type: ToEnvironmentFieldPath
            fromFieldPath: "spec.acl"
            toFieldPath: key1

$ crossplane beta render xr.yaml composition.yaml function.yaml --include-context

...
54 │ ---
55 │ apiVersion: render.crossplane.io/v1beta1
56 │ fields:
57 │ apiextensions.crossplane.io/environment: {}
58 │ kind: Context
I'm expecting to see a key1 in apiextensions.crossplane.io/environment, but maybe this is not how it works?

The right approach was to instead define the following:

    - step: patch-and-transform-to-env
      functionRef:
        name: function-patch-and-transform
      input:
        apiVersion: pt.fn.crossplane.io/v1beta1
        kind: Resources
        environment:
          patches:
            - type: FromCompositeFieldPath
              fromFieldPath: spec.acl
              toFieldPath: key1

But it didn't work firstly because not specifying spec.resources wasn't allowed, which shouldn't be the case.
But after faking a resource it was still failing because the environment was missing the gvk we expect and that crossplane would have set if it had passed an environment:

crossplane: error: cannot render composite resource: pipeline step "patch-and-transform-to-env" returned a fatal result: cannot apply the "FromCompositeFieldPath" environment patch at index 0: cannot patch to object: Object 'Kind' is missing in '{"key1":"private"}'

This PR fixes both issues.