Here are two minor bug fixes

[mtalexander]

These are two minor bug fixes in the preferences code. The IsEmpty method is currently unused but when I tried to use it (on a dead end that I since rejected) I discovered that it doesn't work.

The other change fixes things if you store preferences on certain WEBDAV servers. I'm switching to a2hosting.com for my hosting service and I discovered that their WEBDAV server doesn't require authentication for the initial OPTIONS request, and when it responds to the following GET request with a 401 failure it disconnects before Mulberry can do anything about it. I don't know if this is a Mulberry or a server problem (my guess is server), but getting authentication credentials before the OPTIONS request fixes it.

I suspect similar changes should be made to the other WEBDAV based modules but I don't have any easy way to test that.

I'm a novice at using github pull requests. Let me know if there is a problem with this one.