net/http: fix cookie value of "" being interpreted as empty string.

In issue golang#46443, we have established that double-quotes in cookie values
should be kept as part of the value, rather than being discarded.
However, we have missed the edge case of "" until now. This CL fixes
said edge case.

Fixes golang#75244

Change-Id: I627ad2376931514aa5dcc8961ad804e42b7d9434
Reviewed-on: https://go-review.googlesource.com/c/go/+/700755
Reviewed-by: Nicholas Husin <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Nicholas Husin <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: nicholashusin

src/net/http/cookie.go

@@ -459,9 +459,6 @@ func sanitizeCookieName(n string) string {
 // See https://golang.org/issue/7243 for the discussion.
 func sanitizeCookieValue(v string, quoted bool) string {
 	v = sanitizeOrWarn("Cookie.Value", validCookieValueByte, v)
-	if len(v) == 0 {
-		return v
-	}
 	if strings.ContainsAny(v, " ,") || quoted {
 		return `"` + v + `"`
 	}

src/net/http/cookie_test.go

@@ -530,6 +530,7 @@ func TestCookieSanitizeValue(t *testing.T) {
 		{"a,z", false, `"a,z"`},
 		{",z", false, `",z"`},
 		{"a,", false, `"a,"`},
+		{"", true, `""`},
 	}
 	for _, tt := range tests {
 		if got := sanitizeCookieValue(tt.in, tt.quoted); got != tt.want {