CoerceValue should insert Null for unset attrs

This matches the decoder spec, where a value unset in the configuration
is always Null.

configs/configschema/coerce_value.go

@@ -58,9 +58,7 @@ func (b *Block) coerceValue(in cty.Value, path cty.Path) (cty.Value, error) {
 		switch {
 		case ty.HasAttribute(name):
 			val = in.GetAttr(name)
-		case attrS.Computed:
-			val = cty.UnknownVal(attrS.Type)
-		case attrS.Optional:
+		case attrS.Computed || attrS.Optional:
 			val = cty.NullVal(attrS.Type)
 		default:
 			return cty.UnknownVal(b.ImpliedType()), path.NewErrorf("attribute %q is required", name)

configs/configschema/coerce_value_test.go

@@ -342,6 +342,22 @@ func TestCoerceValue(t *testing.T) {
 			cty.DynamicVal,
 			`.foo: number required`,
 		},
+		"unset computed value": {
+			&Block{
+				Attributes: map[string]*Attribute{
+					"foo": {
+						Type:     cty.String,
+						Optional: true,
+						Computed: true,
+					},
+				},
+			},
+			cty.ObjectVal(map[string]cty.Value{}),
+			cty.ObjectVal(map[string]cty.Value{
+				"foo": cty.NullVal(cty.String),
+			}),
+			``,
+		},
 	}
 
 	for name, test := range tests {