encoding/json: properly unmarshal empty arrays.

The JSON unmarshaller failed to allocate an array when there
are no values for the input causing the `[]` unmarshalled
to []interface{} to generate []interface{}(nil) rather than
[]interface{}{}. This wasn't caught in the tests because Decode()
works correctly and because jsonBig never generated zero-sized
arrays. The modification to scanner_test.go quickly triggers
the error:

without the change to decoder.go, but with the change to scanner_test.go:

$ go test
--- FAIL: TestUnmarshalMarshal (0.10 seconds)
decode_test.go:446:		Marshal jsonBig
scanner_test.go:206:	diverge at 70: «03c1OL6$":null},{"[=» vs «03c1OL6$":[]},{"[=^\»
FAIL
exit status 1
FAIL	encoding/json	0.266s

Also added a simple regression to decode_test.go.

R=adg, dave, rsc
CC=golang-dev
https://golang.org/cl/7196050

src/pkg/encoding/json/decode.go

@@ -742,7 +742,7 @@ func (d *decodeState) valueInterface() interface{} {
 
 // arrayInterface is like array but returns []interface{}.
 func (d *decodeState) arrayInterface() []interface{} {
-	var v []interface{}
+	var v = make([]interface{}, 0)
 	for {
 		// Look ahead for ] - can only happen on first iteration.
 		op := d.scanWhile(scanSkipSpace)

src/pkg/encoding/json/decode_test.go

@@ -239,6 +239,12 @@ var unmarshalTests = []unmarshalTest{
 	{in: `[1, 2, 3]`, ptr: new([1]int), out: [1]int{1}},
 	{in: `[1, 2, 3]`, ptr: new([5]int), out: [5]int{1, 2, 3, 0, 0}},
 
+	// empty array to interface test
+	{in: `[]`, ptr: new([]interface{}), out: []interface{}{}},
+	{in: `null`, ptr: new([]interface{}), out: []interface{}(nil)},
+	{in: `{"T":[]}`, ptr: new(map[string]interface{}), out: map[string]interface{}{"T": []interface{}{}}},
+	{in: `{"T":null}`, ptr: new(map[string]interface{}), out: map[string]interface{}{"T": interface{}(nil)}},
+
 	// composite tests
 	{in: allValueIndent, ptr: new(All), out: allValue},
 	{in: allValueCompact, ptr: new(All), out: allValue},

src/pkg/encoding/json/scanner_test.go

@@ -277,9 +277,6 @@ func genArray(n int) []interface{} {
 	if f > n {
 		f = n
 	}
-	if n > 0 && f == 0 {
-		f = 1
-	}
 	x := make([]interface{}, f)
 	for i := range x {
 		x[i] = genValue(((i+1)*n)/f - (i*n)/f)