TypeList has nil elements when required string attribute is set to empty string

[mrparkers]

I'm working on a custom provider and I'm noticing some strange behavior with a TypeList, in which each element is a schema with a single required string attribute. If you set this attribute to an empty string, the element itself is nil, instead of a map[string]interface{}.

Terraform Version

Terraform v0.12.11

Terraform Configuration Files

Given a schema that looks like this:

"test": {
	Type:     schema.TypeList,
	Optional: true,
	MaxItems: 1,
	Elem: &schema.Resource{
		Schema: map[string]*schema.Schema{
			"a_string": {
				Type:     schema.TypeString,
				Required: true,
			},
		},
	},
},

And HCL that looks like this:

resource "test_resource" "test" {
    test {
        a_string = ""
    }
}

The following provider code produces a panic:

if v, ok := data.GetOk("test"); ok {
	test := v.([]interface{})[0].(map[string]interface{})

	log.Printf("[DEBUG] test: %s", test["a_string"].(string))
}

As v.([]interface{})[0] is nil.

Expected Behavior

In the example above, I expect v.([]interface{})[0] to be a map[string]interface{} with a single key, a_string, with the value of ""

Actual Behavior

In the example above, v.([]interface{})[0] was nil, causing the example code to panic.

Additional Context

I'm a little confused with what the intended behavior is when the end user specifies an empty string for a string attribute that is marked as required. I actually expected the example HCL I posted to fail validation, since I interpreted a "required string" to also be non-empty.

If it is expected for users to be able to use an empty string for required string attributes, then what is the recommended way for a provider developer to ensure that required string attributes are also non-empty? I did a quick search in the validation package and didn't find a helper function that looked like it would help.

Thanks!