[input validation] pcs shall check that location constraint always relates to some nodes in one way or another

```
# pcs constraint location dummy rule id=till-the-new-year score=-INFINITY date gt 2020-01-01
# echo $?
> 0
```

->

```





```

This is a degenerate location constraint that achieves exactly nothing
but more complex overall configuration -- there's no actual "location"
specified in there.

At the very least, this shall raise a warning.

Relevant:
https://bugs.clusterlabs.org/show_bug.cgi?id=5415