SetNew in CustomizeDiff does not trigger diff on computed field when map value is set to empty string

### SDK version

```
{
"Path": "github.com/hashicorp/terraform-plugin-sdk",
"Version": "v1.9.0"
}

```

### Relevant provider source code

```go
package main

import (
"log"

"github.com/google/go-cmp/cmp"
"github.com/hashicorp/terraform-plugin-sdk/helper/schema"
"github.com/hashicorp/terraform-plugin-sdk/plugin"
"github.com/hashicorp/terraform-plugin-sdk/terraform"
)

func main() {
plugin.Serve(&plugin.ServeOpts{
ProviderFunc: Provider})
}

func Provider() terraform.ResourceProvider {
return &schema.Provider{
ResourcesMap: map[string]*schema.Resource{
"mapdiff_test": resourceTest(),
},
}
}

func resourceTest() *schema.Resource {
return &schema.Resource{
Create: resourceTestCreate,
Read: resourceTestRead,
Delete: resourceTestDelete,
Update: resourceTestCreate,
CustomizeDiff: resourceTestDiff,
Schema: map[string]*schema.Schema{
"input": &schema.Schema{
Type: schema.TypeMap,
Required: true,
Elem: &schema.Schema{
Type: schema.TypeString,
},
},
"result": &schema.Schema{
Type: schema.TypeMap,
Computed: true,
Elem: &schema.Schema{
Type: schema.TypeString,
},
},
},
}
}

func resourceTestCreate(d *schema.ResourceData, m interface{}) error {
d.SetId("dooo")

return resourceTestRead(d, m)
}

func resourceTestRead(d *schema.ResourceData, m interface{}) error {
return nil
}

func resourceTestDelete(d *schema.ResourceData, m interface{}) error {
return nil
}

func resourceTestDiff(d *schema.ResourceDiff, m interface{}) error {
d.SetNew("result", d.Get("input"))

o, n := d.GetChange("result")

log.Printf(cmp.Diff(o, n))

return nil
}
```

### Terraform Configuration Files
```hcl
// Example to reproduce.
resource "mapdiff_test" "foo" {
input = {
"baz" = "hahn"
"a" = ""
}
}

// Real world example.
provider "tls" {
version = "~> 2.1"
}

resource "tls_private_key" "example" {
algorithm = "ECDSA"
ecdsa_curve = "P384"
}

resource "mapdiff_test" "foo" {
input = {
"foo" = tls_private_key.example.private_key_pem
"baz" = "hahn"
}
}
```

### Debug Output

https://gist.github.com/invidian/4f1d9d0b6f74f352aad892dab857a8fa

### Expected Behavior

When using `SetNew` from `schema.ResourceDiff` on fields which are `TypeMap` and `Computed`, Terraform should properly write them and show them in diff, not ignore them.

### Actual Behavior

Field `input`, containing user input properly shows diff and gets updated in the state, but field `result` gets ignored and it's never corrected. This might be problematic, when one needs a Map, where the value can actually be an empty string.

Related upstream issue https://github.com/flexkube/libflexkube/issues/48#issuecomment-605352824.

### Steps to Reproduce

1. Save provider code to `main.go` file.
2. Build using `go build -o terraform-provider-diffmap`.
3. Create `main.tf` file with following content:
```hcl
resource "mapdiff_test" "foo" {
input = {
"baz" = "hahn"
"a" = ""
}
}
```
4. Run `terraform init`.
5. Run `terraform apply -auto-approve`.
6. Inspect created Terraform state with `cat terraform.tfstate`. Example output:
```json
{
"version": 4,
"terraform_version": "0.12.24",
"serial": 38,
"lineage": "9f56ba63-345b-ca92-75c4-7e9cbd2d5da1",
"outputs": {},
"resources": [
{
"mode": "managed",
"type": "mapdiff_test",
"name": "foo",
"provider": "provider.mapdiff",
"instances": [
{
"schema_version": 0,
"attributes": {
"id": "dooo",
"input": {
"a": "",
"baz": "hahn"
},
"result": {
"baz": "hahn"
}
},
"private": "bnVsbA=="
}
]
}
]
}
```
7. What actual output should be:
```json
{
"version": 4,
"terraform_version": "0.12.24",
"serial": 38,
"lineage": "9f56ba63-345b-ca92-75c4-7e9cbd2d5da1",
"outputs": {},
"resources": [
{
"mode": "managed",
"type": "mapdiff_test",
"name": "foo",
"provider": "provider.mapdiff",
"instances": [
{
"schema_version": 0,
"attributes": {
"id": "dooo",
"input": {
"a": "",
"baz": "hahn"
},
"result": {
"a": "",
"baz": "hahn"
}
},
"private": "bnVsbA=="
}
]
}
]
}
```

### Context

With [terraform-provider-flexkube](https://github.com/flexkube/libflexkube/tree/master/cmd/terraform-provider-flexkube), I take user configuration, including TLS certificates and then transpile it to more complex structure, which is written to Terraform state to provide user friendly diff. Unfortunately, currently re-generating the certificates triggers inconsistent state, as certificate content is not known while planning. I would expect computed fields which has value set to empty string (`""`) to be seen in diff as "Known after apply", instead of just being ignored. Using `SetNewComputed` on sub-field produces error too.

With "real world example", running Terraform twice or with proper `-target` serves as a workaround. To trigger inconsistent plan issue, one needs to run `terraform taint tls_private_key.example` and then `terraform apply`.

If this is like this by design, how can I avoid getting inconsistent plan in such case?

### References
- Might be related to #282

Start with the ResourceDiff.SetNew and SetNewComputed behavior in the SDK's helper/schema package, then reproduce the issue using main.go and main.tf. Compare the generated terraform.tfstate with the expected result and add coverage showing that a computed TypeMap retains the key whose value is an empty string.

Written by the indexing model from the issue text.