fmt: fix go syntax formatting of []byte(nil)

Fixes #7639.

LGTM=rsc
R=r, adg, rsc
CC=golang-codereviews
https://golang.org/cl/81240043

src/pkg/fmt/fmt_test.go

@@ -403,6 +403,8 @@ var fmtTests = []struct {
 	{"%#v", "foo", `"foo"`},
 	{"%#v", barray, `[5]fmt_test.renamedUint8{0x1, 0x2, 0x3, 0x4, 0x5}`},
 	{"%#v", bslice, `[]fmt_test.renamedUint8{0x1, 0x2, 0x3, 0x4, 0x5}`},
+	{"%#v", []byte(nil), "[]byte(nil)"},
+	{"%#v", []int32(nil), "[]int32(nil)"},
 
 	// slices with other formats
 	{"%#x", []int{1, 2, 15}, `[0x1 0x2 0xf]`},

src/pkg/fmt/print.go

@@ -523,6 +523,15 @@ func (p *pp) fmtString(v string, verb rune, goSyntax bool) {
 func (p *pp) fmtBytes(v []byte, verb rune, goSyntax bool, typ reflect.Type, depth int) {
 	if verb == 'v' || verb == 'd' {
 		if goSyntax {
+			if v == nil {
+				if typ == nil {
+					p.buf.WriteString("[]byte(nil)")
+				} else {
+					p.buf.WriteString(typ.String())
+					p.buf.Write(nilParenBytes)
+				}
+				return
+			}
 			if typ == nil {
 				p.buf.Write(bytesBytes)
 			} else {