interp: let HandlerContext.Stdin be nil again when there is no stdin

We recently changed Runner.stdin from an io.Reader to an *os.File.
One unintended consequence of this change is that HandlerContext.Stdin
is filled directly from that value, so when the file was nil,
we would now fill Stdin with a typed nil, which is not nil.

This new behavior was unintentional and entirely confusing. Avoid it,
and add a comment to ensure we don't fall into that again.

interp/runner.go

@@ -241,10 +241,12 @@ func (r *Runner) handlerCtx(ctx context.Context) context.Context {
 	hc := HandlerContext{
 		Env:    &overlayEnviron{parent: r.writeEnv},
 		Dir:    r.Dir,
-		Stdin:  r.stdin,
 		Stdout: r.stdout,
 		Stderr: r.stderr,
 	}
+	if r.stdin != nil { // do not leave hc.Stdin as a typed nil
+		hc.Stdin = r.stdin
+	}
 	return context.WithValue(ctx, handlerCtxKey{}, hc)
 }