Accept objects with DynamicPseudoType with null values

.changelog/116.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+Fixed a panic when using DynamicPseudoType attributes in an object.
+```

tftypes/object.go

@@ -183,7 +183,7 @@ func valueFromObject(types map[string]Type, optionalAttrs map[string]struct{}, i
 				if v.Type() == nil {
 					return Value{}, NewAttributePath().WithAttributeName(k).NewErrorf("missing value type")
 				}
-				if v.Type().Is(DynamicPseudoType) && v.IsKnown() {
+				if v.Type().Is(DynamicPseudoType) && v.IsKnown() && !v.IsNull() {
 					return Value{}, NewAttributePath().WithAttributeName(k).NewErrorf("invalid value %s for %s", v, v.Type())
 				} else if !v.Type().Is(DynamicPseudoType) && !v.Type().UsableAs(typ) {
 					return Value{}, NewAttributePath().WithAttributeName(k).NewErrorf("can't use %s as %s", v.Type(), typ)

tftypes/value_msgpack_test.go

@@ -447,6 +447,21 @@ func TestValueFromMsgPack(t *testing.T) {
 			value: NewValue(DynamicPseudoType, nil),
 			typ:   DynamicPseudoType,
 		},
+		"object-dynamic-null": {
+			hex: "81a161c0",
+			value: NewValue(Object{
+				AttributeTypes: map[string]Type{
+					"a": DynamicPseudoType,
+				},
+			}, map[string]Value{
+				"a": NewValue(DynamicPseudoType, nil),
+			}),
+			typ: Object{
+				AttributeTypes: map[string]Type{
+					"a": DynamicPseudoType,
+				},
+			},
+		},
 		"dynamic-unknown": {
 			hex:   "d40000",
 			value: NewValue(DynamicPseudoType, UnknownValue),