EnvironmentConfig with empty data field cause errors

We should modify the mergeEnvConfigsData function to explicitly skip EnvironmentConfigs that have a nil data field. According to the EnvironmentConfig definition, the data field is optional:

```
type EnvironmentConfig struct {
metav1.TypeMeta `json:",inline"`
metav1.ObjectMeta `json:"metadata,omitempty"`

// The data of this EnvironmentConfig.
// This may contain any kind of structure that can be serialized into JSON.
// +optional
Data map[string]extv1.JSON `json:"data,omitempty"`
}
```

https://pkg.go.dev/github.com/crossplane/crossplane/apis/apiextensions/v1alpha1#EnvironmentConfig

Currently, when an EnvironmentConfig with a nil data field is processed, the function attempts to get the data field value which will lead to an (unexpected) error. We should explicitly skip these configs to ensure proper handling of empty EnvironmentConfigs.

# Proposed Change

Add a check to skip EnvironmentConfigs with nil data fields

https://github.com/crossplane-contrib/function-environment-configs/blob/f25285a0d0b0c13e1f3ed463d2d61278147f3a03/fn.go#L335

```go
func mergeEnvConfigsData(configs []unstructured.Unstructured) (map[string]interface{}, error) {
merged := map[string]interface{}{}
for _, c := range configs {
// changes start
if c.Object["data"] == nil {
continue // skip if no data field
}
// changes end
data := map[string]interface{}{}
if err := fieldpath.Pave(c.Object).GetValueInto("data", &data); err != nil {
return nil, errors.Wrapf(err, "cannot get data from environment config %q", c.GetName())
}

merged = mergeMaps(merged, data)
}
return merged, nil
}
```

# Proposed Test

``` go
...
"MergeNormalAndEmptyEnvironmentConfigs": {
reason: "The Function should merge a normal EnvironmentConfig with an empty one",
args: args{
req: &fnv1.RunFunctionRequest{
Meta: &fnv1.RequestMeta{Tag: "hello"},
Input: resource.MustStructJSON(`{
"apiVersion": "template.fn.crossplane.io/v1beta1",
"kind": "Input",
"spec": {
"environmentConfigs": [
{
"type": "Reference",
"ref": {
"name": "normal-env-config"
}
},
{
"type": "Reference",
"ref": {
"name": "empty-env-config"
}
}
]
}
}`),
ExtraResources: map[string]*fnv1.Resources{
"environment-config-0": {
Items: []*fnv1.Resource{
{
Resource: resource.MustStructJSON(`{
"apiVersion": "apiextensions.crossplane.io/v1beta1",
"kind": "EnvironmentConfig",
"metadata": {
"name": "normal-env-config"
},
"data": {
"key1": "value1",
"key2": "value2"
}
}`),
},
},
},
"environment-config-1": {
Items: []*fnv1.Resource{
{
Resource: resource.MustStructJSON(`{
"apiVersion": "apiextensions.crossplane.io/v1beta1",
"kind": "EnvironmentConfig",
"metadata": {
"name": "empty-env-config"
}
}`),
},
},
},
},
},
},
want: want{
rsp: &fnv1.RunFunctionResponse{
Meta: &fnv1.ResponseMeta{Tag: "hello", Ttl: durationpb.New(response.DefaultTTL)},
Results: []*fnv1.Result{},
Requirements: &fnv1.Requirements{
ExtraResources: map[string]*fnv1.ResourceSelector{
"environment-config-0": {
ApiVersion: "apiextensions.crossplane.io/v1beta1",
Kind: "EnvironmentConfig",
Match: &fnv1.ResourceSelector_MatchName{
MatchName: "normal-env-config",
},
},
"environment-config-1": {
ApiVersion: "apiextensions.crossplane.io/v1beta1",
Kind: "EnvironmentConfig",
Match: &fnv1.ResourceSelector_MatchName{
MatchName: "empty-env-config",
},
},
},
},
Context: &structpb.Struct{
Fields: map[string]*structpb.Value{
FunctionContextKeyEnvironment: structpb.NewStructValue(resource.MustStructJSON(`{
"apiVersion": "internal.crossplane.io/v1alpha1",
"kind": "Environment",
"key1": "value1",
"key2": "value2"
}`)),
},
},
},
},
},
...
```