return a nil flatmap when presented with a NullVal

A nil flatmap will be encoded as a NullVal of the correct type. When
Converting a NullVal back to a flatmap, return nil immediately rather
than attempting to build the values.

config/hcl2shim/flatmap.go

@@ -21,6 +21,10 @@ import (
 // so the given value must not have any maps of complex types or the result
 // is undefined.
 func FlatmapValueFromHCL2(v cty.Value) map[string]string {
+	if v.IsNull() {
+		return nil
+	}
+
 	if !v.Type().IsObjectType() {
 		panic(fmt.Sprintf("HCL2ValueFromFlatmap called on %#v", v.Type()))
 	}

config/hcl2shim/flatmap_test.go

@@ -228,6 +228,14 @@ func TestFlatmapValueFromHCL2(t *testing.T) {
 				"foo.0.bap.%": UnknownVariableValue,
 			},
 		},
+		{
+			cty.NullVal(cty.Object(map[string]cty.Type{
+				"foo": cty.Set(cty.Object(map[string]cty.Type{
+					"bar": cty.String,
+				})),
+			})),
+			nil,
+		},
 	}
 
 	for _, test := range tests {