cue: decode empty list as empty slice instead of slice-typed nil

These three cases weren't consistent, since using cue.Value.Decode
on an empty interface type as a destination would result in nil:

	{
		value: `[]`,
		dst:   new(interface{}),
		want:  ([]interface{})(nil),
	}, {
		value: `[]`,
		dst:   new([]interface{}),
		want:  []interface{}{},
	}, {
		value: `[]`,
		dst:   new([]int),
		want:  []int{},
	}

Now all result in an empty slice, which is consistent and intuitive.

Closes #2471 as merged as of commit f96a18c.

Signed-off-by: Artem V. Navrotskiy <bozaro@yandex.ru>
Change-Id: I040c09343df12a153e6b2ee645ec9fd118b524ee
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1167647
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>

cue/decode.go

@@ -280,6 +280,9 @@ func (d *decoder) interfaceValue(v Value) (x interface{}) {
 		for list.Next() {
 			a = append(a, d.interfaceValue(list.Value()))
 		}
+		if a == nil {
+			a = []interface{}{}
+		}
 		x = a
 
 	case StructKind:

cue/decode_test.go

@@ -231,6 +231,10 @@ func TestDecode(t *testing.T) {
 				`,
 		dst: &S{},
 		err: "Decode: x: cannot use value 1 (type int) as (string|bytes)",
+	}, {
+		value: `[]`,
+		dst:   new(interface{}),
+		want:  []interface{}{},
 	}}
 	for _, tc := range testCases {
 		t.Run(tc.value, func(t *testing.T) {