Ignore empty folders with invalid names

**Describe the bug**
Creating a new folder using Windows Explorer and the renaming it gives a lot of pointless warnings in the APL session:
```
]link.create c:\tmp\statfns
Linked: #.statfns ←→ "c:\tmp\statfns"
```
(create a new folder using Windows explorer)
```
Link Warning: ⎕SE.Link.Notify: changed c:/tmp/statfns/New folder: invalid name defined by file: c:/tmp/statfns/New folder
Link Warning: Unable to read "c:/tmp/statfns/New folder": c:/tmp/statfns/New folder: Invalid file or directory
```
(rename it to "sub"):
```
Link Warning: ⎕SE.Link.Notify: renamed c:/tmp/statfns/sub: updating previously linked #.statfns.subnamespace #.statfns.New folder not found. Loading #.statfns.sub from c:/tmp/statfns/sub
```
**Expected behaviour**
Ignore such folders, just say nothing.