Optional+Computed attribute of type string can't be force zeroed

### SDK version

```
v1.4.0
```

### Relevant provider source code

I just write a dummy provider to better illustrate my question.

Below code is the schema definition, there defined 3 different attributes:

- name: primitive type of string
- age: primitive type of int
- aliases: aggregate type of list

```go
Schema: map[string]*schema.Schema{
"name": &schema.Schema{
Type: schema.TypeString,
Optional: true,
Computed: true,
},
"age": &schema.Schema{
Type: schema.TypeInt,
Optional: true,
Computed: true,
},
"aliases": &schema.Schema{
Type: schema.TypeList,
Optional: true,
Computed: true,
Elem: &schema.Schema{
Type: schema.TypeString,
},
},
},
```

### Terraform Configuration Files

```hcl
resource "example_server" "my-server" {
name = "foo"
age = 10
aliases = ["a", "b"]
}

```
### Steps to Reproduce

1. `terraform apply` with the config i attached above
2. modify the config to be as below (force zero each attribute):

```
resource "example_server" "my-server" {
name = ""
age = 0
aliases = []
}
```

3. `terraform plan`

### Expected Behavior

```
~ resource "example_server" "my-server" {
~ age = 10 -> 0
~ aliases = [
- "a",
- "b",
]
id = "1.2.3.5"
~ name = ""
}

```

### Actual Behavior

```
~ resource "example_server" "my-server" {
~ age = 10 -> 0
~ aliases = [
- "a",
- "b",
]
id = "1.2.3.5"
name = "foo"
}

```

The `name` attribute doesn't honour the force zero operartion.

Start by reproducing the issue with the provided Optional+Computed schema and configuration, then trace how the SDK handles an empty string for the name attribute during planning. Done means a regression test demonstrates that changing name from a nonempty value to "" is preserved in the plan, consistently with age and aliases.

Written by the indexing model from the issue text.