redo generates empty files for virtual targets

From the README.md:

> When a target makes no output, no target file is created.

However,

```
$ cat clean.do
#!/usr/bin/env bash

rm -rf .tmp
$ redo clean
$ ls -la clean*
-rw------- 1 user user   0 May 29 22:53 clean
-rw-rw-r-- 1 user user 143 May 29 22:53 clean.do
```