fixes #580 tcell custom tty on Windows will not work

Applications will need to supply a working TTY of course.

tscreen_stub.go

@@ -1,7 +1,7 @@
 //go:build js || plan9 || windows
 // +build js plan9 windows
 
-// Copyright 2021 The TCell Authors
+// Copyright 2022 The TCell Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use file except in compliance with the License.
@@ -22,5 +22,11 @@ package tcell
 // that we can obtain with the console API present on Windows.
 
 func (t *tScreen) initialize() error {
-	return ErrNoScreen
+	if t.tty == nil {
+		return ErrNoScreen
+	}
+	// If a tty was supplied (custom), it should work.
+	// Custom screen implementations will need to provide a TTY
+	// implementation that we can use.
+	return nil
 }