Work around problems with autorandr's completion when using `bashcompinit`

Work around problems with autorandr's completion when using `bashcompinit`

On ZSH the easiest approach to get bash completions running is to use
`bashcompinit`. Unfortunately it seems as there are slight differences
between ZSH and bash which cause problems when using that feature.

I had to change the following things:

* Replace the `-n` check with a length check of the AR directory array:
  ZSH adds an empty string at first if no system directory for autorandr
  was found. Checking if there's at least a single element works around
  this problem.

* Silence errors from `find`. The empty string at first causes a
  `directory not found error` during the completion.

It can be tested on ZSH after running the following command:

```
$ autoload -U +X bashcompinit && bashcompinit && source ./contrib/bash_completion/autorandr
```

Tested with zsh 5.7.1 (x86_64-pc-linux-gnu).