GCG-enabled node types don't allow empty location

The GCG form says that setting "Required accuracy" to "Unknown location" is supposed to make the location not required, but in fact I find that if I don't set a location (for example, on my calendar entry type that *sometimes* needs a location, but not always), I get the error messages "The Longitude must be a number" and "The Latitude must be a number" and cannot save/update the item.

So here is a little patch that corrects that.

By the way, awesome module! I was so happy to find this after a lot of misery with the gmap/location set of modules that are currently in significant disarray. Your module works wonderfully for my needs!