GetOk output for null string field and empty string field are identical making it impossible to tell if a value is to be un-set or ignored.

Not sure if bug or feature but this has been kinda frustrating. I have an API that accepts a string field, and as far as requests go, it is valid to send a payload without the field, the field as an empty string (to unset the existing value) and of course the payload with a string.

Passing a zero value such as an empty string to un-set a field is impossible to handle due to the `GetOk` function returning identical responses for a field set to `""` and a field set to `null` or omitted entirely.

Am I doing this wrong or is handling values that are zero values in Go unsupported?

### Terraform Version
```
Terraform v0.14.4
```

### Terraform Configuration Files
```terraform
resource "dummy_resource" "test1" {
typeList_resource {
strField = ""
otherField = "stuff"
}
}
resource "dummy_resource" "test2" {
typeList_resource {
otherField = "stuff"
}
}
resource "dummy_resource" "test3" {
typeList_resource {
strField = "stuff"
otherField = "stuff"
}
}
...
```

### Debug Output
Placing a logger on line 546 of `resource_data.go` inside the `func (d *ResourceData) get(addr[]string, source, getSource) getResult {}` method yields the following:

```
log.Println("Value:", result.Value, "zero_or_schema:" result.ValueOrZero(schema), "is nil?" result.Value == nil)
```

```
Value: zero_or_schema: is nil? true // test 1 - unexpected (expected is nil to be false and result.Value to be "")
Value: zero_or_schema: is nil? true // test 2 - expected (field omitted so should be nil)
Value: stuff zero_or_schema: stuff is nil? false. // test 3 - expected (field present and has non-empty value)
```

Also logging output of `GetOk("typeList_resource.0.strField)` yields

```
GetOK: false // test 1 - unexpected (expected true since this field is set, but to the zero value for string)
GetOK: false // test 2 - expected (field not set so should get false for second variable)
GetOK: stuff true. // test 3 - expected (field present and got true for second variable)
```

### Crash Output
No Crash

### Expected Behavior
Expected `GetOk("typeList_resource.0.strField")` to return `"", true` for the empty string and ` false` for the missing field

### Actual Behavior
`GetOk("typeList_resource.0.strField")` returns `"", false` in both cases. Now there's no way to tell which resource had the `strField` set to `""` and which was missing entirely.

### Steps to Reproduce
1. `terraform init`
2. `terraform apply`

### Additional Context
N/A

### References
None

Start in resource_data.go at ResourceData.get and inspect the behavior used by GetOk for the three configurations in the issue. Reproduce the empty, omitted, and non-empty string cases with the provided Terraform examples, then verify that the completed behavior distinguishes an explicitly empty string from a missing or null field.

Written by the indexing model from the issue text.