Ignore folders which are empty and names which are not valid in APL

[mkromberg]

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.

[mkromberg]

Empty folders are ignored by Git anyway.