bug#60100: 29.0.60; project-current is not resilient

[Juri Linkov]

Is this intentional that

```
(project-current nil "/tmp/nonexistent/")
```

raises the error

  Debugger entered--Lisp error: (file-missing "Opening directory" "No such file 
or directory" "/tmp/nonexistent")

instead of silently returning nil.  Because when called with
an existing non-project dir, it just returns nil:

```
(project-current nil "/tmp/") => nil
```