syscall: restore EscapeArg behavior for empty string

Accidentally broken by CL 259978.

For #41825

Change-Id: Id663514e6eefa325faccdb66493d0bb2b3281046
Reviewed-on: https://go-review.googlesource.com/c/go/+/260397
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Alex Brainman <alex.brainman@gmail.com>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/exec_windows.go

@@ -24,6 +24,9 @@ var ForkLock sync.RWMutex
 // - finally, s is wrapped with double quotes (arg -> "arg"),
 //   but only if there is space or tab inside s.
 func EscapeArg(s string) string {
+	if len(s) == 0 {
+		return `""`
+	}
 	for i := 0; i < len(s); i++ {
 		switch s[i] {
 		case '"', '\\', ' ', '\t':

src/syscall/exec_windows_test.go

@@ -0,0 +1,43 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syscall_test
+
+import (
+	"syscall"
+	"testing"
+)
+
+func TestEscapeArg(t *testing.T) {
+	var tests = []struct {
+		input, output string
+	}{
+		{``, `""`},
+		{`a`, `a`},
+		{` `, `" "`},
+		{`\`, `\`},
+		{`"`, `\"`},
+		{`\"`, `\\\"`},
+		{`\\"`, `\\\\\"`},
+		{`\\ `, `"\\ "`},
+		{` \\`, `" \\\\"`},
+		{`a `, `"a "`},
+		{`C:\`, `C:\`},
+		{`C:\Program Files (x32)\Common\`, `"C:\Program Files (x32)\Common\\"`},
+		{`C:\Users\Игорь\`, `C:\Users\Игорь\`},
+		{`Андрей\file`, `Андрей\file`},
+		{`C:\Windows\temp`, `C:\Windows\temp`},
+		{`c:\temp\newfile`, `c:\temp\newfile`},
+		{`\\?\C:\Windows`, `\\?\C:\Windows`},
+		{`\\?\`, `\\?\`},
+		{`\\.\C:\Windows\`, `\\.\C:\Windows\`},
+		{`\\server\share\file`, `\\server\share\file`},
+		{`\\newserver\tempshare\really.txt`, `\\newserver\tempshare\really.txt`},
+	}
+	for _, test := range tests {
+		if got := syscall.EscapeArg(test.input); got != test.output {
+			t.Errorf("EscapeArg(%#q) = %#q, want %#q", test.input, got, test.output)
+		}
+	}
+}