main.go: don't set TTY interface from tty.Open() in case open fails

If tty.Open fails the return tty is still nil, but the interface type will be set; meaning in go that `TTY == nil` will always be false.

fix #7506

main.go

@@ -186,22 +186,20 @@ func wrappedMain() int {
 			return 1
 		}
 	} else {
-		var TTY packer.TTY
+		basicUi := &packer.BasicUi{
+			Reader:      os.Stdin,
+			Writer:      os.Stdout,
+			ErrorWriter: os.Stdout,
+		}
+		ui = basicUi
 		if !inPlugin {
-			var err error
-			TTY, err = tty.Open()
-			if err != nil {
+			if TTY, err := tty.Open(); err != nil {
 				fmt.Fprintf(os.Stderr, "No tty available: %s\n", err)
 			} else {
+				basicUi.TTY = TTY
 				defer TTY.Close()
 			}
 		}
-		ui = &packer.BasicUi{
-			Reader:      os.Stdin,
-			Writer:      os.Stdout,
-			ErrorWriter: os.Stdout,
-			TTY:         TTY,
-		}
 	}
 	// Create the CLI meta
 	CommandMeta = &command.Meta{