Repository: kubernetes
Issue: Empty pod env var always causes server-side apply conflict
Labels: kind/bug,sig/api-machinery,triage/accepted

### What happened?

I applied a manifest that contains an empty env var in a `PodTemplate` (like `env: [{ name: foo, value: "" }]`). Then, I tried to apply the same spec, with a different field-manager. This produced a conflict:

```
error: Apply failed with 1 conflict: conflict with "kubectl": .spec.containers[name="foo"].env[name="foo"].value
Please review the fields above--they currently have other managers. Here
are the ways you can resolve this warning:
* If you intend to manage all of these fields, please re-run the apply
  command with the `--force-conflicts` flag.
* If you do not intend to manage all of the fields, please edit your
  manifest to remove references to the fields that should keep their
  current managers.
* You may co-own fields by updating your manifest to match the existing
  value; in this case, you'll become the manager if the other manager(s)
  stop managing the field (remove it from their configuration).
```

### What did you expect to happen?

I expect the apply to succeed without a conflict, and the field to be co-owned by both managers (since they are both applying the same value).

### How can we reproduce it (as minimally and precisely as possible)?

Apply the following manifest twice:

1. `kubectl apply --server-side -f ./repro.yaml`
2. `kubectl apply --server-side -f ./repro.yaml --field-manager test`

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: foo
spec:
  containers:
    - name: foo
      image: busybox
      args: ["sleep", "1000"]
      env:
        - name: foo
          value: ""  # works as expected if this field is removed, or set to a non-empty value
```

### Anything else we need to know?

I don't think this is `kubectl`-specific, it happens when using `client-go` directly as well.

The behaviour around env values is a bit strange in general - if I specify an empty env var value, the value is listed under `managedFields`, but not actually returned in the response object:

```yaml
apiVersion: v1
kind: Pod
metadata:
  managedFields:
  - apiVersion: v1
    fieldsType: FieldsV1
    fieldsV1:
      f:spec:
        f:containers:
          k:{"name":"foo"}:
            .: {}
            f:args: {}
            f:env:
              k:{"name":"foo"}:
                .: {}
                f:name: {}
                f:value: {}  # value is a managed field
            f:image: {}
            f:name: {}
  name: foo
  namespace: default
spec:
  containers:
  - args:
    - sleep
    - "1000"
    env:
    - name: foo  # but there is no value in the returned object!
    image: busybox
    name: foo
```

### Kubernetes version

<details>

```console
> kubectl version
Client Version: v1.31.0
Kustomize Version: v5.4.2
Server Version: v1.31.0
```

</details>


### Cloud provider

<details>
N/A, using kind
</details>


### OS version

<details>

N/A, using kind

</details>


### Install tools

<details>
kind
</details>


### Container runtime (CRI) and version (if applicable)

<details>

</details>


### Related plugins (CNI, CSI, ...) and versions (if applicable)

<details>

</details>